I finally fixed it!!
Since OneDrive was mention in other replies and I too use OneDrive, I went into OneDrive and unchecked "Save space and download files as I need them". As soon as I did that (no restart needed), the thumbnails loaded for every image in every folder!!
Can't believe it was that simple, yet the Microsoft Team couldn't/didn't figure it out and post the fix on any of the 50 or so threads with the same problem, and that I've scoured looking for the fix!
SMH . . . Another corporation that our money and support has made too big to be bothered with us.