I just experienced something along these lines ... may or may not be related. I had a one-instrument-track project with a 32-bit synth loaded. Tried to do a replace synth (from the context menu clicking on the mini-keyboard icon in the track header). The screen went mostly white and X1 would not close. Killed X1 and it would refused to start again until after I rebooted.
If anyone who's experiencing these shutdown problems could see if the same thing happens on replace-synth on their system, it might give the bakers a little bit better clue as to what the problem is.