diff --git a/lib/ext/Syncroton/Command/Sync.php b/lib/ext/Syncroton/Command/Sync.php
--- a/lib/ext/Syncroton/Command/Sync.php
+++ b/lib/ext/Syncroton/Command/Sync.php
@@ -964,6 +964,12 @@
                 }
                 
                 $totalChanges += $collectionChanges;
+
+                // If the client resent an old sync-key, we should still respond with the latest sync key
+                if (isset($collectionData->syncState->counterNext)) {
+                    //TODO we're not resending the changes in between, but I'm not sure we have to. 
+                    $collectionData->syncState->counter = $collectionData->syncState->counterNext;
+                }
                 
                 // increase SyncKey if needed
                 if ((
diff --git a/lib/kolab_sync_backend_state.php b/lib/kolab_sync_backend_state.php
--- a/lib/kolab_sync_backend_state.php
+++ b/lib/kolab_sync_backend_state.php
@@ -41,6 +41,7 @@
      */
     public function create($object, $keep_previous_state = true)
     {
+        unset($object->counterNext);
         $object = parent::create($object);
 
         if ($keep_previous_state !== true) {
@@ -184,6 +185,7 @@
         if ($next > $sync_key) {
             // We store the clientIdMap with the "next" sync state, so we need to copy it back.
             $state->clientIdMap = $states[$next]->clientIdMap;
+            $state->counterNext = $next;
         } else {
             // finally delete all entries marked for removal in syncroton_content table
             $retryCounter = 0;