Happend several times now:
Playing a mission and from one second to the next SK has been closed and Steam shows me the usual advertisement you see when you end the game. Upon login you are back in the ready room and unable to get back to the mission you just played. It's not a disconnect from the server, or a lag induced kick, but an actual shut down of the game.
It happens only rarely but most often in a boss fight, for whatever reason.