Unknown Error prevents Multiplayer logon

[Hurum]

Whenever I attempt to logon to multiplayer, I get a window pop up with "Unknown Error" and a tick button that takes me back to the login screen. I have checked the password is correct

This first started happening on my home computer, after I successfully logged on to process a turn on my friends PC. Weird thing is I can still login on his PC. I have the steam version if that makes any difference.