Though I'm not the thread starter, I can confirm that. We have the same problem here. Strangest things happen... I can connect to other players, but I cannot connect to my friend. We both DO own legit copies of the game.
Even on separate internet connections, we either get "The host has left the game" (when I try to enter HIS game), or Connection problems 1-2 we he joins my game. At least JOINING works, but only in "one direction".