Metal gear solid peace walker

[Cordon]

Hello! Lately I've been trying to get MGS PW's coop to work so I can play with my friend. The problem is, that it doesn't work. He can join the lobby just fine but when we both start mission the screen goes black and we can still hear the menu music. The loading icon isn't showing too. Then after a few seconds an error message pops up. Something along the lines of: "You have been disconnected from the network."
And then it makes us go back to the menu. The interesting thing is that we tried versus ops and it worked perfectly no problems. I'm not sure what the problem is at all but it doesn't seem to be lying in us since versus works but coop does not. Please help, I really wanna play this game with my friend!