Yeah, im having the same problem. My buddy and i bought our own copies of the game (retail). I can see his COOP server, but it tells me the session is no longer available if i try to join it. It finally let me in the lobby when he invited me to the game though the friends list, but it still wouldnt let me play! Said my cd key was already in use. I can join any multiplayer game, even his if he puts one up! Just not COOP. Really dissapointing, since that was one of the main reasons i bought it. Surely there is something I can do to make it work.
Im running version 1.3.1080 by the way. I'll let you guys know if i have any luck.