Multiplayer doesn´t work

[[IS]Odinn]

I and my friend can´t play in multiplayer, every time we make a game, two headed giant or whatever, there comes this annoying error "can´t connect to host or session has ended" or something like that. It doesn´t matter if it´s me who is making the game or my friend. We used to be able to play in multiplayer, but one day, it just stopped working.

We´ve bot tried to check the content of the game files and/or uninstalling and installing it again, but it doesn´t matter.

Any suggestions on how to fix this?