Taasz/Woof

Nice, narrowing it down!

Next thing I would check is your configured DNS servers on the desktop, it needs to have only the pihole IP, if it has any other servers configured that is likely the issue.

Also worth pinging the pihole IP next time it’s not working, to check if it’s actually a networking issue instead of a DNS issue.

I don’t think it’s a problem with PiHole realistically, it sounds more like for some reason your DNS requests are not getting to PiHole.

So I would start with checking if the request is reaching PiHole.

Next time it breaks, before restarting networkmanager, go check the pihole requests log and see if your DNS queries are even showing up there.

If they are, what does pihole show it’s returning for the query, is it the correct IP?

If that’s working properly then I would check if you can ping the server by IP directly, make sure that connection is working.