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
@@ -545,19 +545,23 @@
             } 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)) {
+                // Report not found folders as such.
+                // Outlook refuses to remove some folders, and keeps requesting them.
+                // If we instead reported a folder hierarchy change, 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;
+                $collection = $collections->appendChild($this->_outputDom->createElementNS('uri:AirSync', 'Collection'));
+                $collection->appendChild($this->_outputDom->createElementNS('uri:AirSync', 'SyncKey', $collectionData->syncKey));
+                $collection->appendChild($this->_outputDom->createElementNS('uri:AirSync', 'CollectionId', $collectionData->collectionId));
+                $collection->appendChild($this->_outputDom->createElementNS('uri:AirSync', 'Status', self::STATUS_OBJECT_NOT_FOUND));
+
+                continue;
             }
-        }
 
-        foreach($this->_collections as $collectionData) {
             $collectionChanges = 0;
             
             /**