Some images missing in Media library

Something is seriously wrong with my media library. I probably created this problem, but don’t know how and really need help to fix it.

THIS IS WHAT IS HAPPENING:

MOST of my images show up in Media Library but too many show up only as a grey box. (It happens with both recent images and really old ones.) This occurs both in the Media Library in the Admin panel (Grid view and list view) and when I am editing a post and want to edit/insert one of these existing but missing images.
I know the image is there, it’s just that Media Library seems unable to either find it or render it for the thumbnail.

In Media Library, if I click on the grey box where the image should be, it takes me to the Attachment Details page, with no image showing. But then if I click on “edit Image,” “view attachment page,” or “edit more details,” the image comes right up. So, the image is there, it just can’t be viewed until I get into one of those underlying pages!

Images in posts look perfectly normal both for users and in the edit page.

However, when editing a post:
? If I click on one of these “missing” images with the edit pencil symbol, it takes me into the image details page with the image showing in the corner. But the Display settings don’t link to a URL or an image size (full, thumbnail, etc.).
? If I go to Add Media to insert one of these images into the page, the Add Media page shows a grey box.
o If I click on Edit Image, the Edit Image page opens with the image showing.
o If I hit Insert, the image shows up in the post (sometimes after a super, super long wait and maxing out my server CPU) And now if I click on the image in the post I’m editing it shows both the sizes available and links to the media file and attachment page that were missing before. HOWEVER, it still shows up as a grey box in the Add Media page and the Media Library.

Pulling up an image to view using Media Library – in the library or editing in a post – often spins and spins before loading the image. Usually it pushes physical memory and CPU usage on my (shared) server to 100% (That’s never been an issue before.)

THIS IS WHAT I HAVE DONE TO TRY TO FIX IT:

I deactivated all of my plug-ins except Classic Editor to test this and completely uninstalled most of my media-related plug-ins. No change in the problem.

I tried regenerating the missing thumbnails individually with Regenerate Thumbnails and, while it sometimes brings up a single image in Media Library, that image disappears again as soon as I open another image or navigate away.

I tried Media Sync. It doesn’t recognize these as missing images.

I have tested this with the Twenty Twenty theme and these images are still missing from the Library, so I don’t think it’s an issue with my Catch Responsive theme.

I have checked that all permissions are set to 0644.

I reloaded WordPress.

This is a very old site with many, many photos. I’ve been trying for the last month to eliminate thousands and thousands of unused thumbnails and duplicate files. In doing that, I have used Media Cleaner, Regenerate Thumbnails, Stop Generating Image Sizes, and Media Library Assistant. I’ve also done two file and database restores during this process. I’m guessing by the timing this is related to a combination of Media Cleaner and the file/database restore.

Looking for ideas for what to do next to either fix it or at least determine what is causing this.

The page I need help with: [log in to see the link]