Spooky... As it often is the case with Windows...
Thank you for your reply and time.
When I uncheck "Show error description", the app returns with the message "The target segment contains a grammar error".
When I check this case, the message is "!!!" (and not "???" anymore, strange...). Checking or unchecking that case doesn't really change the problem, I'm afraid... :-(
I also disabled the UAC to see, but got the same messages, unfortunately.
I also downloaded the LanguageTool, but the problem there is that I get the message Cannot connect to server... Also :-(.
Note that I installed MS Grammar Checker on a brand new PC that I installed 2 weeks ago, so this PC is still quite clean regarding Windows 10 (1607), I think...
So there must be a reason why Grammar Checker doesn't show the right error messages on my new PC, I'm afraid. You could if needed connect to my PC for this MS Grammar Checker problem.