Thank you everybody for helping me with forum troubleshooting. The problem has been found. It's memory limit. After a few years of use, 512MB RAM + 256MB swap is not enough anymore (I have some suspicions that swap does not kick in, but it's a topic for a different story).
If somebody searches, and no changes plugins, core, or hardware, and you do not have plugins that depend on external services - check if it's a memory issue.
Does someone know a way to create a post-only category?
I am trying to create a category for ban appeals in which users should be able to create a new topic and only see their own topics listed so other users can't see their appeal and if they appealed.
If that is possible, is it possible to make pinned topics readable for everyone?
Otherwise, what would be the best way to achieve something like this?
Should work with localhost but I also prefer to be explicit and specify localhost via IP address. Of course those nginx log tails would have pointed us thusly a long ways back. Checking log files for errors and warnings are step one in troubleshooting 101. 😈