Error downloading slideshow photos

I stumbled on the oddest work around!

I began by selecting a few images at a time and clicking on Slideshow, then I cut down the number until I found one that was causing a problem. Then I tried to find a copy on my computer to see if I could replace it. However I didn't find one so that avenue was a dead end. Next, and I'm not sure why, I selected "revert to original" with the problem photo and lo and behold slideshow worked. So then I did "undo revert to original" and Slideshow worked again.

So as a final test I took a whole folder reverted to original and then undid revert to original and Slideshow worked.

Of course this doesn't explain what the original problem was but hopefully this will help people with the same problem and perhaps Apple Dudes to improve the Photos App.

More details:

After going through all my folders as described above I decided to go through the 27000 photos in the library by selecting several hundred at once and doing the "revert to original" then "undo revert to original" then Slideshow. it took a while but worked but one thing puzzles me. As you can see from the screen grab, photos are being downloaded but where from as I don't use iCloud for storing photos.

Incidentally, prior to these actions, my people section was stuck on scanning photos. This procedure fixed it!

I hope this helps someone.