Guys I solved it by hosting a network with radmin VPN

guttaslymm NAT was moderate

Me and my friend recently tried to play BO2 zombies, but when we try to join each other's parties, we get "Unable to join game session" every time.

To clarify we play in the same room and we have played before and only rarely got this error, but since about a month ago we get it every time without fail. I've read the documentation and watched tutorials, they were all useless.