I followed the instructions to repair permissions. No change to the white thumbnails.
I exported a "white" photo to my desktop. It shows up fine there. When I reimport it to Photos, it shows up fine there too.
I'm getting inconsistent results. I took one white thumbnail, double-clicked it, then tried to rotate it. This caused its thumbnail to come back. But I did the same for a second photo, and it didn't work. (My experiment doing this a few days ago was doing some batch rotations, not single rotations.)
Furthermore, I've noticed that many of the "white" ones are taken by my iPhone XR, and it only affects photos. Movies are always fine. So I clicked on one of the white thumbnails and clicked the "i"info button. This is a HEIC photo. But when I export to my desktop it becomes a JPG. I'm not really familiar with HEIC photos. Is it possible Photos is having a problem with HEIC files? (I'm sure it SHOULDN'T have a problem with them, but I'm just guessing at possible causes at this point. I still have a lot of white photos, and remember, they are only white on this machine, not when viewed on iCloud.com.)