AFK in game for too long = ERROR_317000

I walked away from the computer for an hour or so while still being in-game. Upon return, I was still standing in Tristram and attempted to both "Leave Game" and "Exit Diablo 3." Both attempts to quit the game resulted in this error message, and I couldn't leave the game without killing it from the Windows Task Manager:
"Please join a game and try again. (ERROR_317000)"