It happens at the end of a Zone run too, so it's not just a multiplayer issue.

Another glitch: When you filter the online lobbies to show "friends only" it goes blank (despite numerous friends online). Then when you "unfilter" it to show all lobbies, that's blank too. I tried overcoming this by pressing triangle to join a specific friend's game but it just brought up the location menu. No way around it but to restart the game, it's happened a couple of times now.