Photos: Cannot Start Editing. Photos cannot load adjustments for this image.

UPDATE: It's worse than I thought. The image Export Unmodified Original, then opened in Preview, then exported with a new name, then re-imported into Photos app now appears as a Burst photo when I search by filename, one of which is editable, then other has the "Cannot Start Editing" error. But there is no way to select one of the burst photos. Worse yet, there are three photos in the "burst", one of which is a different photo that has the same filename, except it's HEIC instead of JPG. But it's a completely different photo. But when I scroll to find the photo as opposed to searching by filename, it's not considered burst, though has the burst thumbnail appearance. When I right-click to remove 2 of the 3 burst photos, it deletes the whole photo.

Even worse yet, in Finder I duplicated the Export Unmodified Original photo and gave it another name, and imported it into my Photos library. I can't find it, trying search, Imports, scrolling to the date, searching the exact filename. Now I'm really concerned.

Even worse, Export Unmodified Original then imported into another Photos library seems be OK. Didn't even have to Export from Preview. 12,000 photos in my main library, thousands of which have been edited. How screwed am I?

I have daily, weekly, and monthly backup drives using Time Machine.

My MB Pro was serviced two months ago, so I restored from Time Machine backups. There were some problems, so I did a clean new install of Big Sur, and then restored again. So I haven't considered the need for disk cleanup. I did run Disk Utility First Aid, and it completed without incident.

I use shared folders on iCloud. Not the whole library - my two largest libraries are quite large. I kept the Aperture import in a separate Photos library to avoid problems like this - that library seems to be fine. The library I've "protected" from Aperture is the one with problems.

The library is stored on the internal 1TB SSD, formatted as APFS, in the Pictures folder as is Photos default.

I did have a small breakthrough: instead of trying to edit the apparently corrupted photos, I right-click the thumbnail and select Revert to Original - which allows me to then edit. I'm still very concerned about imported photos disappearing, non-burst photos showing the burst themed thumbnail, and three burst images where the third image is a completely different image. I have an IT background, and stuff like this spooks me. So even though I found a workaround for the Cannot Start Editing problem, the other odd behaviors hint at some underlying internal corruption in my Photos library that may really bite me later.

Thanks for your thoughtful questions.