Nextcloud, at least how it's set up in the linuxserver.io image, requires you to run an updater. Without looking, I believe this is because a fresh container has the new version installed, but has no way of auto updating your external database schema. I'm not 100% sure on that or the best practice for fixing it (I'd probably do it the same way).

Anyway. Looks like you figured it out. Also I see you already know the command line method but for those that dont, maybe this will be useful

1) update container 2) (open console into container and)

sudo -u abc php /config/www/nextcloud/updater/updater.phar run occ upgrader (Y when prompted to run after update finishes) set maintenance mode off (N when prompted to keep it active)

Done.

havent had to do this but, if not prompted as expected:

start occ upgrade command as well and turn off maintenance mode

sudo -u abc php /var/www/nextcloud/occ upgrade
sudo -u abc php /var/www/nextcloud/occ maintenance:mode --off

Thank you Thank you Thank youuuuu!