diff --git a/lib/ext/Syncroton/Command/Ping.php b/lib/ext/Syncroton/Command/Ping.php
--- a/lib/ext/Syncroton/Command/Ping.php
+++ b/lib/ext/Syncroton/Command/Ping.php
@@ -82,8 +82,9 @@
                     } catch (Syncroton_Exception_NotFound $senf) {
                         if ($this->_logger instanceof Zend_Log) 
                             $this->_logger->debug(__METHOD__ . '::' . __LINE__ . " " . $senf->getMessage());
-                        $status = self::STATUS_FOLDER_NOT_FOUND;
-                        break;
+                        // Outlook refuses to removed some folders, and keeps requesting them.
+                        // Without this we end up in an endless loop: https://bifrost.kolabsystems.com/T399575
+                        continue;
                     }
                 }
                 $this->_device->pingfolder = serialize(array_keys($folders));
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
@@ -470,7 +470,10 @@
                 foreach($this->_collections as $collectionData) {
                     // continue immediately if folder does not exist 
                     if (! ($collectionData->folder instanceof Syncroton_Model_IFolder)) {
-                        break 2;
+                        // Ignore the not found folder.
+                        // Outlook refuses to remove some folders, and keeps requesting them.                                       
+                        // Without this we end up in an endless loop: https://bifrost.kolabsystems.com/T399575
+                        continue;
                         
                     // countinue immediately if syncstate is invalid
                     } elseif (! ($collectionData->syncState instanceof Syncroton_Model_ISyncState)) {
@@ -545,23 +548,21 @@
             } while (Syncroton_Server::validateSession() && time() - $intervalStart < $this->_heartbeatInterval - (Syncroton_Registry::getPingTimeout() + 10));
         }
 
-        // First check for folders hierarchy changes
-        foreach ($this->_collections as $collectionData) {
-            if (! ($collectionData->folder instanceof Syncroton_Model_IFolder)) {
 
-                // Without this Outlook will next do a FolderSync with the same syncKey,
-                // and thus not detect any changes.
-                $this->_syncStateBackend->resetState($this->_device, 'FolderSync');
 
+
+
+        foreach($this->_collections as $collectionData) {
+            if (! ($collectionData->folder instanceof Syncroton_Model_IFolder)) {
+                // Ignore the not found folder.
+                // Outlook refuses to remove some folders, and keeps requesting them.
+                // Without this we end up in an endless loop: https://bifrost.kolabsystems.com/T399575
                 if ($this->_logger instanceof Zend_Log)
-                    $this->_logger->warn(__METHOD__ . '::' . __LINE__ . " Detected a folder hierarchy change on {$collectionData->collectionId}.");
+                    $this->_logger->warn(__METHOD__ . '::' . __LINE__ . " Detected an unknown folder, skipping {$collectionData->collectionId}.");
 
-                $sync->appendChild($this->_outputDom->createElementNS('uri:AirSync', 'Status', self::STATUS_FOLDER_HIERARCHY_HAS_CHANGED));
-                return $this->_outputDom;
+                continue;
             }
-        }
 
-        foreach($this->_collections as $collectionData) {
             $collectionChanges = 0;
             
             /**