Plutonium

Update: I found this little error message in the console, does it break anything?

Whenever my friends create a private server and I try to join, I get this error:
It basically means that I can't join they're session, no more info given. Me and my friends have already done the steps named here. I genuinely don't even know what to do, since there are no other topics on the forum that lists the issue I have right here (or i just didn't find any).