Web UI gives "Server Error (500)"

Everything else seems to be working - ssh, smb, plex, etc. It happened once a week or so ago and I rebooted and it went a way, but since it’s come back I figured I’d look closer. Any ideas what might cause this?

I did recently upgrade the CPU in the server from a Celeron to an i5, so that could be related.

The nginx access.log shows the 500 responses, but error.log is empty.

/var/log/messages has a few interesting things like apfd getting permission denied trying to call get_tm_bandsize on macbook backups, and systemd starting sessions and creating user slices, but nothing that looks relevant to the web UI.

What logs would be useful?