When I join a game, I have no problem and connection is established with the host. If a player joins, my lobby freezes for a while and when it comes back to life, I am connected with the player but no more with the host.
The problem occures only whith some players (a lot though, this is very annoying), it is not systematic when I join a game...
I did not have the problem with my precedent modem/router. So I think it might be a port forwarding problem but I opened the 6112 (which is the one used by my game) and it doesnt change anything.
Maybe it is a known problem but I did not find anything here or on the wiki.