Unexpected happenings:
There are details about this in the Sidecar Files for Movies thread I also started, so here's the short version:
I haven't changed any of my anti-virus settings (still using Norton, Windows Defender is still deactivated, not uninstalled).
I created a Movies Test Library, it has five movies in it. The Library file itself is located on my server computer local hard drive, but not in the default location MC uses.
I have been having trouble with NAS Library, which is the Main Library MC creates when installed. I renamed it, but left it in the default folder where MC creates it.
I tried a few tests to "fix" problematic Movie files in Movies Test Library, they were unsuccessful. I then tried Get TV and Movie Info (which has caused problems during the past few days). Everything worked as it should. The Cover Art I chose displays properly and the Movie plays properly. When I check my folder using Windows Explorer there is an .mkv, .jpg and .xml file. All is good.
I next switched Libraries to use NAS Library. I repeated my experiment using Get TV and Movie Info. The Cover Art I chose downloaded and displayed properly, but the video file was destroyed (155 KB, wouldn't play).
I can think of two possibilities:
NAS Library is corrupted somehow.
Norton is messing with activity between my NAS and the Library folder (the default folder MC creates for the Main Library).
I guess I'll stew for a while about whether to create a new Library in the default folder and try again, or expand the Movie Test Library (which seems to be working properly) and then make that my Main Library.
Of course, I could read the Taming Windows Defender thread and see what Awesome Donkey has to say...