This happens where everything turns black, a white window appears and doesn't display anything. I can click on things but there is no response. I have no choice but to force close it through task manager and somtimes it doesn't reopen recover files and I lose hours of work. This is something that happens often and I don't know the problem.
first, save your fla's with increasing version number so you never lose more than a few minutes of work. ie, save project1_v000.fla, project1_v001.fla, etc
next, see if you can stop the crashes by resetting your an preferences, https://helpx.adobe.com/animate/using/set-preferences.html
Have you tried to upgrade to 21.0.4?