Can't play co-op after latest patch, keep getting ERR5001, help!

Whenever I try to launch a coop campaign with a friend, I get ERR5001 and the game loads but in a completely different map and no scenario settings, just as a normal random map. This only happens when I host the game. If my friend does it and invites me, then everything appears to work so it looks like it is something on my end. Please help!