I have a couple of raspberry pi 4 with 4 GB and Ubuntu installed. One of the changes I did was to have them all auto update nightly. It seems though that one of them is having trouble as I cannot ssh into it and the mediawiki website I set up for documentation is not browsable on my local network. It works if I reboot the raspberry pi again.

What is the proper logs to check? I don't know what is causing it, but just suspect it might be some daemons that hanging because they need to be started. Or something like it.

Try turning off auto update for one night and see if it still crashes?

Otherwise, since Ubuntu uses systemd, any daemons/services crashing should be logged in the journal.

If you can access the machine directly (without rebooting), see what "sudo systemctl status" and "sudo journalctl" say. (You can use "sudo journalctl -u servicename" to see logs related to just that service.)

Otherwise, if you need to reset it before you can login, use "sudo journalctl -b -1" to access logs for the previous boot.

(If that doesn't return anything, man systemd-journald has details of how to enable persistent journal.)

Thanks for the help. I had to restart again. And I might have found the cause of it. The "sudo journalctl -b -1" unfortunately didn't give me any useful info. However I remember that Netdata is installed for server monitoring, and it looks like the memory usage is building up until hitting the 4 GB, and the virtual memory as well. It's mainly the X application that is to blame, but this is a server. And I have an old script looking for available boat places on a website using puppeteer. So it's probably faulty now and isn't closing correctly. The cron job is running it every 15 minutes without checking if the previous process finished. So I will comment it out and see if it solves it.