Fatal error: Uncaught Error: Interface 'Sabre\CalDAV\Backend\BackendInterface' not found in /home/malert/WebDAV/vendor/sabre/dav/lib/CalDAV/Backend/SyncSupport.php:21 Stack trace: #0 {main} thrown in /home/malert/WebDAV/vendor/sabre/dav/lib/CalDAV/Backend/SyncSupport.php on line 21