Error during coop play. "Your copy of the game is out of sync, pls try again."

I played 3 quests with a friend without any problem and at the quest "Here they come" when we play for 4-5 mins this error comes out and we return to our capital cities. We tried this quest 3 times but theabove messagekeeps coming. Any solutions?|||Sometimes the client bugs and it does this after a bunch of co-op quests. Both of you should relog and see if that helps? Also I notice when the quest log bugs out then that person will need to relog as well.