Plutonium

Hello everyone, I am trying to play BO2 Multiplayer with a friend, but we keep getting this issue. Basically we are able to join each other in a room, but then, when joining a public server, only the leader of the party can join, and the other player gets the "Party is no longer active" error.
We have the same NAT (Moderate), and we are both in Europe (one in Italy and one in Germany).
If we join the same server by self, we meet in the game, but usually the server put us in the opposite teams, and that's annoying XD.
We can also play Zombie in a custom game with no problem.
Anyone has a solution to this? Thanks in advance!