Apparently today is Sysadmin appreciation day: https://sysadminday.com/
So thank you me for keeping this puppy up and running. I guess it's time for a round of updates, dealing with broken Docker images, checking that backups and restores work and general stuff I've been putting of for far too long.
Aaaaaaaand of course I ended up with this lovely boot error message:
Could not find /vmlinuz-linux