Downloaded Dungeons with Hero yesterday.  Worked fine in single player mode.  We could successfully add local coop players (offline), but as soon as we tried to start playing, the game would freeze on the loading screen and then kick the game back to the XBox home screen menu.  We tried resetting the XBox; we tried using different people as Player 1.  We tried adding different numbers of coop players.  Didn't seem to make a difference.  Same result each time.