This has happened a few times now when I've been playing without any friends in a group. After finishing a mission, I will be returned to the main menu as the server loads a new map, with a pop-up at the top of the screen saying my party is playing a mission. If I click on the Join Group button in that pop-up, I will get an error. This will go on indefinitely.
If I go to Play to cue up for a new game, I get a Waiting For Leader message and am unable to begun matchmaking. I need to close and restart the game to clear the error.