I bought this game dirt cheap at retail with the idea of playing the missions co-op with mah cousin, but it seems no matter what we do, whenever either of us wants to join the game the other hosted, the error "This session is no longer available" pops up. We both set off to google it, and it seems that while this problem is very widespread, no one has a solid solution for it.

Perhaps someone in the massively helpful Giant Bomb community knows something?