diff --git a/app/src/main/java/net/moasdawiki/app/MainActivity.java b/app/src/main/java/net/moasdawiki/app/MainActivity.java
index e9cc3d1..f2bc43d 100644
--- a/app/src/main/java/net/moasdawiki/app/MainActivity.java
+++ b/app/src/main/java/net/moasdawiki/app/MainActivity.java
@@ -159,6 +159,7 @@ public class MainActivity extends AppCompatActivity {
                     // determine URL path
                     Uri uri = request.getUrl();
                     String encodedUrl = uri.toString();
+                    //noinspection CharsetObjectCanBeUsed
                     String url = URLDecoder.decode(encodedUrl, "UTF-8");
                     if (!url.startsWith(SERVER_BASE_URL)) {
                         return null;
@@ -184,7 +185,7 @@ public class MainActivity extends AppCompatActivity {
                     }
                     InputStream responseData = new ByteArrayInputStream(response.getContent());
                     return new WebResourceResponse(mimeType, "UTF-8", responseData);
-                } catch (IOException e) {
+                } catch (Exception e) {
                     e.printStackTrace();
                 }
 
@@ -345,24 +346,33 @@ public class MainActivity extends AppCompatActivity {
      * This method is run in an asynchronous background thread.
      */
     private void runSynchronizationWithServer() {
-        int filesCount;
+        SynchronizeWikiClient.SyncResult syncResult;
         try {
-            filesCount = synchronizeWikiClient.synchronizeRepository(this::syncProgress);
+            syncResult = synchronizeWikiClient.synchronizeRepository(this::syncProgress);
         } catch (ServiceException e) {
             Log.e(TAG, "Error synchronizing repository with server", e);
-            runOnUiThread(() -> showToast(getString(R.string.settings_synchronize_failed)));
+            runOnUiThread(() -> showToast(getString(R.string.synchronize_failed)));
             return;
         }
 
-        if (filesCount > 0) {
-            runOnUiThread(() -> showToast(getString(R.string.settings_synchronize_successful, filesCount)));
+        if (!syncResult.isSessionValid()) {
+            runOnUiThread(() -> showToast(getString(R.string.synchronize_session_not_valid)));
+        }
+        else if (!syncResult.isSessionAuthorized()) {
+            runOnUiThread(() -> showToast(getString(R.string.synchronize_session_not_authorized)));
+        }
+        else if (syncResult.isSyncFailed()) {
+            runOnUiThread(() -> showToast(getString(R.string.synchronize_failed)));
+        }
+        else if (syncResult.getFileCount() > 0) {
+            runOnUiThread(() -> showToast(getString(R.string.synchronize_successful, syncResult.getFileCount())));
 
             WikiEngineApplication app = (WikiEngineApplication) getApplication();
             app.resetServices();
 
             CalendarSyncAdapter.requestCalendarSync(this);
         } else {
-            runOnUiThread(() -> showToast(getString(R.string.settings_synchronize_not_necessary)));
+            runOnUiThread(() -> showToast(getString(R.string.synchronize_not_necessary)));
         }
 
         runOnUiThread(this::hideProgressBar);
diff --git a/app/src/main/java/net/moasdawiki/app/SettingsFragment.java b/app/src/main/java/net/moasdawiki/app/SettingsFragment.java
index 7dfa1d1..0ebb671 100644
--- a/app/src/main/java/net/moasdawiki/app/SettingsFragment.java
+++ b/app/src/main/java/net/moasdawiki/app/SettingsFragment.java
@@ -185,10 +185,10 @@ public class SettingsFragment extends PreferenceFragmentCompat implements Shared
             return;
         }
 
-        boolean success = synchronizeWikiClient.createAndCheckSession();
+        SynchronizeWikiClient.SessionStatus sessionStatus = synchronizeWikiClient.createAndCheckSession();
         updateStatusText();
 
-        if (!success) {
+        if (!sessionStatus.isValid()) {
             showToast(getString(R.string.settings_search_failed));
         }
     }
diff --git a/app/src/main/java/net/moasdawiki/app/SynchronizeWikiClient.java b/app/src/main/java/net/moasdawiki/app/SynchronizeWikiClient.java
index 99c6499..3a065f0 100644
--- a/app/src/main/java/net/moasdawiki/app/SynchronizeWikiClient.java
+++ b/app/src/main/java/net/moasdawiki/app/SynchronizeWikiClient.java
@@ -39,6 +39,7 @@ import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 
 import java.io.ByteArrayOutputStream;
+import java.io.IOException;
 import java.io.InputStream;
 import java.io.OutputStream;
 import java.math.BigInteger;
@@ -47,14 +48,14 @@ import java.net.InetAddress;
 import java.net.NetworkInterface;
 import java.net.SocketException;
 import java.net.URI;
+import java.net.URL;
 import java.nio.charset.StandardCharsets;
 import java.security.SecureRandom;
 import java.util.Date;
 import java.util.Enumeration;
 
 /**
- * Sucht einen Wikiserver in Netzwerk und synchronisiert alle Wikidateien
- * im eigenen Repository..
+ * Connects to the configured MoasdaWiki server and downloads the wiki files.
  */
 public class SynchronizeWikiClient {
 
@@ -62,6 +63,10 @@ public class SynchronizeWikiClient {
 
     private static final String PROTOCOL_VERSION = "2.0";
 
+    private static final int CONNECTION_CONNECT_TIMEOUT = 2_000; // 2 seconds
+    private static final int CONNECTION_READ_TIMEOUT = 120_000; // 2 minutes
+    private static final int CONNECTION_RETRIES = 3;
+
     @NotNull
     private final Context mContext;
     @NotNull
@@ -83,13 +88,13 @@ public class SynchronizeWikiClient {
     }
 
     /**
-     * Verbindet sich mit dem Wikiserver. Ist bereits eine Serversession vorhanden, wird diese
-     * weiter verwendet.
+     * Connects with the MoasdaWiki server.
+     * If there is already a valid server session, it is reused.
      */
-    public boolean createAndCheckSession() {
+    public SessionStatus createAndCheckSession() {
         String serverHostPort = getServerHostPort();
         if (serverHostPort == null) {
-            return false;
+            return new SessionStatus(false, false);
         }
 
         PreferenceManager.getDefaultSharedPreferences(mContext);
@@ -97,37 +102,39 @@ public class SynchronizeWikiClient {
         String serverSessionId = preferences.getString(Constants.PREFERENCES_SYNC_SERVER_SESSION_ID, null);
 
         try {
-            // Wenn noch keine Session vorhanden ist, eine erzeugen
+            // If there is no session yet, create a new session
             boolean createSessionCalled = false;
             if (serverSessionId == null) {
                 createSessionCalled = true;
                 createSession(serverHostPort);
             }
 
-            // Ist Session freigeschaltet zur Synchronisierung?
-            boolean[] checkResult = checkSession(serverHostPort);
-            if (checkResult[0]) {
-                return true;
+            // Check existing session if it's still valid
+            SessionStatus sessionStatus = checkSession(serverHostPort);
+            if (sessionStatus.isValid()) {
+                return sessionStatus;
             }
 
-            // Session ist ungültig --> neue Session holen
+            // If session ist invalid, create new session
             if (createSessionCalled) {
-                // createSession nicht erneut aufrufen
-                return false;
+                // don't call createSession() twice
+                return new SessionStatus(false, false);
             }
             createSession(serverHostPort);
 
-            // Session erneut prüfen
-            boolean[] checkResult2 = checkSession(serverHostPort);
-            return checkResult2[0];
+            // Check session again
+            return checkSession(serverHostPort);
         }
         catch (ServiceException e) {
-            return false;
+            return new SessionStatus(false, false);
         }
     }
 
+    /**
+     * Connects with the MoasdaWiki server and creates a new session.
+     */
     private void createSession(@NotNull String serverHostPort) throws ServiceException {
-        // Anfrage schicken
+        // send request
         CreateSessionXml createSessionXml = new CreateSessionXml();
         createSessionXml.version = PROTOCOL_VERSION;
         createSessionXml.clientSessionId = generateSessionId();
@@ -141,11 +148,11 @@ public class SynchronizeWikiClient {
         String requestXml = generateXml(createSessionXml);
         String responseXml = sendXmlRequest(serverHostPort, "/sync/create-session", requestXml);
 
-        // Antwort auswerten
+        // parse response
         CreateSessionResponseXml response = parseXml(responseXml, CreateSessionResponseXml.class);
         Log.d(TAG, "Current sync session ID '" + response.serverSessionId + "'");
 
-        // Session prüfen
+        // check for session id
         if (response.serverSessionId == null) {
             throw new ServiceException("Didn't get a server session ID");
         }
@@ -157,29 +164,34 @@ public class SynchronizeWikiClient {
         editor.putString(Constants.PREFERENCES_SYNC_SERVER_HOST_DISPLAYNAME, response.serverHost);
         editor.putString(Constants.PREFERENCES_SYNC_SERVER_SESSION_ID, response.serverSessionId);
         editor.putString(Constants.PREFERENCES_SYNC_CLIENT_SESSION_ID, createSessionXml.clientSessionId);
-        // Synchronisierungszeiten können nicht mehr genutzt werden
+        // reset sync data
         editor.remove(Constants.PREFERENCES_SYNC_SERVER_TIME);
         editor.remove(Constants.PREFERENCES_SYNC_SERVER_SESSION_AUTHORIZED);
         editor.apply();
     }
 
+    @NotNull
     private String generateSessionId() {
         return new BigInteger(130, random).toString(32);
     }
 
-    private boolean[] checkSession(@NotNull String serverHostPort) throws ServiceException {
+    /**
+     * Check if our MoasdaWiki server session is valid and authorized.
+     */
+    @NotNull
+    private SessionStatus checkSession(@NotNull String serverHostPort) throws ServiceException {
         SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(mContext);
         String serverSessionId = preferences.getString(Constants.PREFERENCES_SYNC_SERVER_SESSION_ID, null);
         String clientSessionId = preferences.getString(Constants.PREFERENCES_SYNC_CLIENT_SESSION_ID, null);
 
-        // Anfrage schicken
+        // Send request
         CheckSessionXml checkSessionXml = new CheckSessionXml();
         checkSessionXml.version = PROTOCOL_VERSION;
         checkSessionXml.serverSessionId = serverSessionId;
         String requestXml = generateXml(checkSessionXml);
         String responseXml = sendXmlRequest(serverHostPort, "/sync/check-session", requestXml);
 
-        // Antwort auswerten
+        // Parse response
         CheckSessionResponseXml response = parseXml(responseXml, CheckSessionResponseXml.class);
         if (response.valid == null || !response.valid) {
             Log.d(TAG, "Sync server session ID '" + serverSessionId + "' is not valid any more");
@@ -187,7 +199,7 @@ public class SynchronizeWikiClient {
             editor.remove(Constants.PREFERENCES_SYNC_SERVER_SESSION_ID);
             editor.remove(Constants.PREFERENCES_SYNC_CLIENT_SESSION_ID);
             editor.apply();
-            return new boolean[]{ false, false };
+            return new SessionStatus(false, false);
         }
         if (clientSessionId != null && !clientSessionId.equals(response.clientSessionId)) {
             Log.d(TAG, "Sync server authentication failed, client session ID does not match");
@@ -195,37 +207,70 @@ public class SynchronizeWikiClient {
             editor.remove(Constants.PREFERENCES_SYNC_SERVER_SESSION_ID);
             editor.remove(Constants.PREFERENCES_SYNC_CLIENT_SESSION_ID);
             editor.apply();
-            return new boolean[]{ false, false };
+            return new SessionStatus(false, false);
         }
 
         SharedPreferences.Editor editor = preferences.edit();
         boolean authorized = response.authorized != null && response.authorized;
         editor.putBoolean(Constants.PREFERENCES_SYNC_SERVER_SESSION_AUTHORIZED, authorized);
         editor.apply();
-        return new boolean[]{ true, authorized };
+        return new SessionStatus(true, authorized);
     }
 
     /**
-     * Synchronisiert alle Repository-Dateien mit dem Wikiserver. Vorher wird geprüft, ob
-     * die Server-Session noch gültig ist und wir mit demselben Server verbunden sind.
-     *
-     * @return Anzahl der synchronisierten Dateien.
+     * Contains the session status.
      */
-    public int synchronizeRepository(ProgressFeedback feedback) throws ServiceException {
-        boolean sessionValid = createAndCheckSession();
-        if (!sessionValid) {
-            throw new ServiceException("No valid server session found");
+    public static class SessionStatus {
+        /**
+         * Is the MoasdaWiki server session valid?
+         */
+        private final boolean valid;
+
+        /**
+         * Is the session authorized at server side?
+         */
+        private final boolean authorized;
+
+        public SessionStatus(boolean valid, boolean authorized) {
+            this.valid = valid;
+            this.authorized = authorized;
         }
 
+        public boolean isValid() {
+            return valid;
+        }
+
+        public boolean isAuthorized() {
+            return authorized;
+        }
+    }
+
+    /**
+     * Downloads the wiki files from the MoasdaWiki server.
+     * Checks if the session is still valid.
+     */
+    public SyncResult synchronizeRepository(ProgressFeedback feedback) throws ServiceException {
         String serverHostPort = getServerHostPort();
         if (serverHostPort == null) {
-            throw new ServiceException("No wiki server configured");
+            Log.w(TAG, "No server host name configured");
+            return new SyncResult(false, false, true, 0);
+        }
+
+        SessionStatus sessionStatus = createAndCheckSession();
+        if (!sessionStatus.isValid()) {
+            Log.w(TAG, "No valid server session");
+            return new SyncResult(false, false, true, 0);
+        }
+        if (!sessionStatus.isAuthorized()) {
+            Log.w(TAG, "Server session not authorized");
+            return new SyncResult(true, false, true, 0);
         }
 
         SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(mContext);
         String serverSessionId = preferences.getString(Constants.PREFERENCES_SYNC_SERVER_SESSION_ID, null);
         if (serverSessionId == null) {
-            throw new ServiceException("Not server session ID found");
+            Log.w(TAG, "No server session available");
+            return new SyncResult(false, false, true, 0);
         }
         long lastSyncServerTimeMs = preferences.getLong(Constants.PREFERENCES_SYNC_SERVER_TIME, 0);
         Date lastSyncServerTime = null;
@@ -247,7 +292,7 @@ public class SynchronizeWikiClient {
         Log.d(TAG, "Downloading " + fileCount + " files from server");
         if (fileCount == 0) {
             // no files to download, cancel process
-            return 0;
+            return new SyncResult(true, true, false, 0);
         }
 
         for (int i = 0; i < fileCount; i++) {
@@ -274,7 +319,37 @@ public class SynchronizeWikiClient {
             editor.apply();
         }
 
-        return fileCount;
+        return new SyncResult(true, true, false, fileCount);
+    }
+
+    public static class SyncResult {
+        private final boolean sessionValid;
+        private final boolean sessionAuthorized;
+        private final boolean syncFailed;
+        private final int fileCount;
+
+        public SyncResult(boolean sessionValid, boolean sessionAuthorized, boolean syncFailed, int fileCount) {
+            this.sessionValid = sessionValid;
+            this.sessionAuthorized = sessionAuthorized;
+            this.syncFailed = syncFailed;
+            this.fileCount = fileCount;
+        }
+
+        public boolean isSessionValid() {
+            return sessionValid;
+        }
+
+        public boolean isSessionAuthorized() {
+            return sessionAuthorized;
+        }
+
+        public boolean isSyncFailed() {
+            return syncFailed;
+        }
+
+        public int getFileCount() {
+            return fileCount;
+        }
     }
 
     private void downloadFileFromServer(@NotNull String serverHostPort, @NotNull String serverSessionId, @NotNull String filePath) throws ServiceException {
@@ -372,11 +447,7 @@ public class SynchronizeWikiClient {
     }
 
     /**
-     * Schickt einen XML-Request und liest die XML-Antwort ein.
-     *
-     * @param urlPath    HTTP-URL, nicht null
-     * @param requestXml XML-Anfrage, nicht null.
-     * @return XML-Antwort, nicht null.
+     * Sends an XML request and reads the XML response.
      */
     @NotNull
     private String sendXmlRequest(@NotNull String serverHostPort, @NotNull String urlPath, @NotNull String requestXml) throws ServiceException {
@@ -385,32 +456,11 @@ public class SynchronizeWikiClient {
             Log.d(TAG, "Request to " + url + ": " + truncateLogText(requestXml, 200));
             byte[] requestBytes = requestXml.getBytes(StandardCharsets.UTF_8);
 
-            URI uri = new URI(url);
-            HttpURLConnection conn = (HttpURLConnection) uri.toURL().openConnection();
-            conn.setRequestMethod("POST");
-            conn.setRequestProperty("Content-Type", "text/xml");
-            conn.setRequestProperty("Content-Length", Integer.toString(requestBytes.length));
-            conn.setUseCaches(false);
-            conn.setDoOutput(true);
-            conn.setDoInput(true);
-            conn.setConnectTimeout(1000); // 1 Sekunde
-            conn.setReadTimeout(10000); // 10 Sekunden
-            conn.connect();
+            byte[] responseBytes = sendBinaryRequestWithRetries(new URI(url).toURL(), requestBytes);
 
-            OutputStream out = conn.getOutputStream();
-            out.write(requestBytes);
-            out.flush();
-
-            InputStream in = conn.getInputStream();
-            ByteArrayOutputStream byteStream = new ByteArrayOutputStream(in.available());
-            int bytesRead;
-            byte[] buffer = new byte[1024];
-            while ((bytesRead = in.read(buffer)) != -1) {
-                byteStream.write(buffer, 0, bytesRead);
-            }
-
-            String responseXml = byteStream.toString("UTF-8");
-            Log.d(TAG, "Response: " + truncateLogText(responseXml, 400));
+            //noinspection CharsetObjectCanBeUsed
+            String responseXml = new String(responseBytes, "UTF-8");
+            Log.d(TAG, "Response: " + truncateLogText(responseXml, 100));
             return responseXml;
         } catch (Exception e) {
             Log.e(TAG, "Error sending XML request", e);
@@ -418,6 +468,44 @@ public class SynchronizeWikiClient {
         }
     }
 
+    private byte[] sendBinaryRequestWithRetries(@NotNull URL url, byte[] requestBytes) throws ServiceException {
+        for (int i = 1; i <= CONNECTION_RETRIES; i++) {
+            try {
+                return sendBinaryRequest(url, requestBytes);
+            }
+            catch (Exception e) {
+                Log.d(TAG, "Error sending request to MoasdaWiki server in attempt " + i + " of " + CONNECTION_RETRIES, e);
+            }
+        }
+        throw new ServiceException("Error sending request to MoasdaWiki server for " + CONNECTION_RETRIES + " times, failed");
+    }
+
+    private byte[] sendBinaryRequest(@NotNull URL url, byte[] requestBytes) throws IOException {
+        HttpURLConnection conn = (HttpURLConnection) url.openConnection();
+        conn.setRequestMethod("POST");
+        conn.setRequestProperty("Content-Type", "text/xml");
+        conn.setRequestProperty("Content-Length", Integer.toString(requestBytes.length));
+        conn.setUseCaches(false);
+        conn.setDoOutput(true);
+        conn.setDoInput(true);
+        conn.setConnectTimeout(CONNECTION_CONNECT_TIMEOUT);
+        conn.setReadTimeout(CONNECTION_READ_TIMEOUT);
+        conn.connect();
+
+        OutputStream out = conn.getOutputStream();
+        out.write(requestBytes);
+        out.flush();
+
+        InputStream in = conn.getInputStream();
+        ByteArrayOutputStream byteStream = new ByteArrayOutputStream(in.available());
+        int bytesRead;
+        byte[] buffer = new byte[1024];
+        while ((bytesRead = in.read(buffer)) != -1) {
+            byteStream.write(buffer, 0, bytesRead);
+        }
+        return byteStream.toByteArray();
+    }
+
     private String truncateLogText(String logText, int maxLength) {
         if (logText.length() <= maxLength) {
             return logText;
@@ -443,7 +531,7 @@ public class SynchronizeWikiClient {
             XmlParser xmlParser = new XmlParser(logger);
             return xmlParser.parse(xml, xmlBeanType);
         } catch (ServiceException e) {
-            logger.write("Failed to parse XML for class " + xmlBeanType.getSimpleName() + ", try class ErrorResponseXml", e);
+            Log.d(TAG, "Failed to parse XML for class " + xmlBeanType.getSimpleName() + ", try class ErrorResponseXml", e);
             // Versuche eine Fehlerantwort zu parsen
             XmlParser xmlParser = new XmlParser(logger);
             ErrorResponseXml errorResponseXml = xmlParser.parse(xml, ErrorResponseXml.class);
diff --git a/app/src/main/res/values-de/strings.xml b/app/src/main/res/values-de/strings.xml
index 64e1c3c..3c99a58 100644
--- a/app/src/main/res/values-de/strings.xml
+++ b/app/src/main/res/values-de/strings.xml
@@ -28,7 +28,9 @@
     <string name="settings_status_server_connected">Am Server angemeldet</string>
     <string name="settings_status_server_authorization_mission">Erfordert Berechtigung am Server</string>
     <string name="settings_search_failed">Keinen Wikiserver gefunden!</string>
-    <string name="settings_synchronize_successful">%1$d Dateien erfolgreich synchronisiert</string>
-    <string name="settings_synchronize_not_necessary">Dateien sind bereits aktuell</string>
-    <string name="settings_synchronize_failed">Synchronisierung nicht möglich, bitte überprüfen Sie die Einstellungen und den Status!</string>
+    <string name="synchronize_successful">%1$d Dateien erfolgreich synchronisiert</string>
+    <string name="synchronize_not_necessary">Dateien sind bereits aktuell</string>
+    <string name="synchronize_session_not_valid">Keine Verbindung zum MoasdaWiki-Server vorhanden! Bitte überprüfe die Einstellungen und den Status!</string>
+    <string name="synchronize_session_not_authorized">Fehlende Berechtigung beim MoasdaWiki-Server! Bitte schalte den Zugang für die App beim Server frei!</string>
+    <string name="synchronize_failed">Synchronisierung nicht möglich, bitte überprüfe die Einstellungen und den Status!</string>
 </resources>
diff --git a/app/src/main/res/values/strings.xml b/app/src/main/res/values/strings.xml
index 57ae670..4ddebe9 100644
--- a/app/src/main/res/values/strings.xml
+++ b/app/src/main/res/values/strings.xml
@@ -29,7 +29,9 @@
     <string name="settings_status_server_connected">Connected to server</string>
     <string name="settings_status_server_authorization_mission">Needs authorization at server</string>
     <string name="settings_search_failed">No Wiki server found!</string>
-    <string name="settings_synchronize_successful">Successfully synchronized %1$d files</string>
-    <string name="settings_synchronize_not_necessary">All files are up-to-date</string>
-    <string name="settings_synchronize_failed">Synchronization failed, please check settings and status!</string>
+    <string name="synchronize_successful">Successfully synchronized %1$d files</string>
+    <string name="synchronize_not_necessary">All files are up-to-date</string>
+    <string name="synchronize_session_not_valid">No connection to MoasdaWiki server available! Please check settings and status!</string>
+    <string name="synchronize_session_not_authorized">Connection to MoasdaWiki server not authorized! Please authorize at server side first!</string>
+    <string name="synchronize_failed">Synchronization failed, please check settings and status!</string>
 </resources>