Okay, in the meantime I was able to fix it. What I did:
- uninstalled nextcloud via UMC
- installed nextcloud 26.0. by issuing
univention-app install nextcloud=26.0.10-0 - issued
sudo -u www-data php /var/www/html/occ maintenance:mode --off - tried to login: no passwords were accepted
- removed nextcloud from the command line
univention-app remove nextcloud - re-installed nextcloud 26.0. by issuing
univention-app install nextcloud=26.0.10-0 - login possible, filesharing worked, calendar app was missing, calendar clients could not connect
- logged in as administrator and loaded the calendar app from the nextcloud store
Now everything works again and nextcloud 26.0 is shown as installed (App-center update available to 27.1.6-0 - should I dare … ?)