BFV "Game Disconnected" When trying to enter a game.

Friend of mine has a similar problem (PC). He cannot join a game by himself, he will get disconnected. The he needs to restart the game.

If he joins my squad and follows me on a server, there is a 50% chance that he will not disconnect this time.

Problem appeared some weeks ago, reasons unknown. We reinstalled the game, we checked router and manually opened the recommended ports, cleared the cache...  nothing helps.