Can those who have crossword problems please tell me which antivirus you use? I have a suspicion, but need some feedback to confirm it.
Also, if you are affected by the problem, try the following: Load a crossword and leave the window open (for at least half an hour) while you do something else. Then try again. I had the problem once, forgot to close the window and about an hour later, the crossword worked (as did others I played afterwards).
McAfee Anti-Virus for me, but as I said, it only seems to be a problem w/ the MS Edge browser. I think there's a Flash problem there all-around since the last Win10 update. Crosswords on Firefox were wonky for a couple of months too (the last letter of each word entry would not show up until I went to the next word, and the green highlight strip was 1/2 missing), but somehow it's been corrected now.
I did try your experiment of opening a puzzle in MS Edge and leaving it for over an hr., but still a no go.