Fatal Error connecting to a NextCloud oinstance

Using a NextCloud instance as WebDAV-Storage for remote backups results in a

Fatal error: Type of Sabre\DAV\Xml\Service::$elementMap must be array (as in class Sabre\Xml\Service) in /var/www/ud01_320/html/OV_live/wp-content/plugins/xcloner-backup-and-restore/vendor/sabre/dav/lib/DAV/Xml/Service.php on line 17

Checked several NextCloud instances (my own one, the one of our organization, the one of my employer), none did work. Occurred the first time about 3 months ago. Worked some years before.