Nextcloud account creation failed: Calendar make it fail

My CalDAV account via NC stopped syncing approx. two weeks ago. Connection to NC is fine. Looks like there is a prob between local account on my XA2 and the native Jolla calendar.

I asked as well here

and, initially, in German:
https://jollausers.de/thread/854-xa2-kalender-caldav-nextcloud-sync-schlägt-fehl/#post8747

Any ideas if something was changed in the background? Debug help?