Im in the same boat u guys are in. Just one single guy in my squad i cannot play games with in the same room. Whenever im in the room and he tries to join, we get that damn message. But if i leave the room and he tries to enter the room, he is successful. Thats still bad for me because then i cannot enter the room without getting that message.
We used to play battlefield 2 togather and it worked fine its something with the damn game. I even sent an email to my ISP and im still awaiting my response.
My NAT is open so its not that