Failed to Join Error

Me and a friend have been playing this game for like 2 weeks up until yesterday. Now we get a failed to join after a 10 second loading screen. If we join each other from an invite, we get an instant failed to join. Anybody had this issue? We think its because my NAT type is moderate, and his is strict, but that doesn't make any sense because we've been able to play thus far. It has only now been saying his NAT type is strict at the bottom of his screen, and we think that that could possibly be the issue? Please help, we really like this game and want to continue playing it. We've tried everything besides port forwarding, mostly because neither of us can port forward.