Problems after container update

Silly me. I upgraded my Nextcloud docker container to 28.x and can’t step back - got a message that my data was ported to v28 and that downgrade isn’t supported.
Now I’m stuck. Nextcloud won’t boot up, got 2 errors.

First: a 500 is thrown. I seems that the DB connection is refused (which was working so far with v.26x). My DB is on a mariadb container, located in the same stack. Already tested that the user has all permissions (show grants for nextclouduser). Confusing :frowning: Did anyone met this problem?

The other message is

{"reqId":"ao8yAAsfm3vnj7GuDh7d","level":3,"time":"2024-02-09T22:05:49+00:00","remoteAddr":"","user":"--","app":"PHP","method":"GET","url":"/","message":"Class OCA\\GroupFolders\\Helper\\LazyFolder contains 2 abstract methods and must therefore be declared abstract or implement the remaining methods (OCP\\Files\\Folder::searchBySystemTag, OCP\\Files\\FileInfo::getMetadata) at /var/www/html/custom_apps/groupfolders/lib/Helper/LazyFolder.php#32","userAgent":"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/ Safari/537.36","version":"","data":{"app":"PHP"}}

Help or hints would be greatly appreciated.

Well wtf… (sorry). Since I’m convinced that the DB was setup correctly, I played around a bit. At one point I set maintenence mode to true in config.php - and suddenly a had output in the browser. Puh.
But it only stated “I’m in maintenance mode”, nothing else.
Then I’ve removed ‘apps_paths’ from config.php, refreshed the page several times. And then I added ‘apps_paths’ again. Refreshed the page and got the “Do-the-update”-button. Seems to work now. But… wtf…??
How was I supposed to get this update button?
(Before I got there accidentally the whole page didn’t work).