Haven't played the game in a while, so this error may be update related. But I verified game files through Steam and everything seems to be alright.
When I try to play co-op I keep getting a weird error I've never encountered before. First, when I'm in the menu no friends show up in the co-op list, and no friends show up in game. When I try to join a friend's game through the Steam overlay nothing happens. When I try and accept an invite from the menu an error message pops up saying "This invitation is for a different game than the one you're currently playing. Please close the