As always the nextcloud update failed for me…
After a quick search I found this post. Seems like using mariadb:latest is not a good idea anymore. After switching to mariadb:10.5 and manually turning the maintenance mode off I could proceed the update process.
$ docker exec --user www-data nextcloud-app_1 php /var/www/html/occ maintenance:mode --off
Nextcloud or one of the apps require upgrade - only a limited number of commands are available
You may use your browser or the occ upgrade command to do the upgrade
Maintenance mode disabled