It was a custom-built LXC container I built on an Alpine Linux base.

I used the Alpine packages to upgrade it, then afterwards I used the Nextcloud admin scripts to migrate the schema, apps & plugins.

Biggest clusterfuck I’ve ever dealt with. Not doing that again.

TBF the Alpine-setup probably made everything worse, and that’s a lesson learned, but I’m just fed up and can’t bother setting up a new instance now.