Myself and someone else on the steam forums have run into a bug where the "end turn" button gets stuck on something else like "choose production" or "city-state message." There doesn't appear to be any way to clear it and end your turn.
Also, trying to quit and rejoin the game in progress doesn't work at all. You'll rejoin to the lobby while the rest of the players are still in game. You can chat with everyone (but the host apparently), but they can't move their units anymore.