Have also removed the Arma3.cfg file totally and re-launched just in case. Same issue.
This may not help but there's a part of the loading process where when you play in window mode the window goes into a "not responding" state - just before the GUI normally launches and shows the game menus etc.. It seems to be right at that point that it's hanging and not doing whatever it used to do.
I know - amateur descriptions. Sorry. Report attached anyway.