photos library shows blanks after updating the library What happened

opened my fairly big (1TB) photos library (stored on an external drive, backup/copy on a different external drive)
Photos started to update the library (no problems, took about half an hour or less)
Browsed through the photos, realized that there were many blanks:


In full screen mode, the images appear blank:

This is the same for all media, i.e. Live Photos, videos and portraits. However, all media is still in the library package (opened the files in Preview, no problem there) and all metadata (location, tags, lens info, date) is fine, too. Further, these photos are not camera-, location-, face-detection- or file name-specific. They seem to be totally random, however all photos taken before march 2015 are fine (I am probably reading into this, but Photos.app was introduced April 2015).
What I have tried so far:

Repair the library
Restart the Mac
Reset SMC and NVRAM/PRAM
Run first aid on both external disks
Copied the backup onto my work drive, opened it and it was updated as well. No change there (the blank images seem to be the same for both libraries)
Waited about half an hour to display a blank image, no change
Tried to export the original image and video, both fail with unknown error (4), yet some Portraits seem to be exporting fine
Tried to export (not the original) fails with The process couldn't be finished. (Cocoa-error-1), this time including Portraits

More info about the system

MBP 15&quot; from 2017 with macOS BigSur 11.2.1

Both external drives have about 1TB of free storage, their total size is 4TB

I have opened a library after the Big Sur update without problems, but I am not sure if I have opened this library on Big Sur

The backup I have copied has definitely not yet been opened on Big Sur

iCloud is not enabled on this library and it is not the system library


My question is: Does anyone know how I can effectively get these blanks to disappear, without having to manually reimport all photos?

I have a couple more options/ideas up my sleeve, however, they will take a lot of time, so maybe someone here already knows a better/faster fix/idea than the following:

Copy my currently not-updated backup to an older Catalina Mac, hope it can be opened there
As suggested in this thread delete the corrupted libraries, then copy them back (really unsure how this would help though)
Update macOS to 11.2.3



Update #1

I have updated macOS to version 11.2.3 to no avail.

Update #2

I have talked to Apple support. Further things that we tried, all to no avail:

Change the library to the system library
Create another admin account and  open the library there
Start into safe mode
Imported problematic images into a new library, there they show up as normal


Update #3

It seems like I actually have already opened this library on BigSur as I just tried to open it with a Catalina Mac, which resulted in an error message, saying I would need to update the OS.

Update #4

I talked to Apple support again. The engineers didn't come up with anything/they don't really offer database repair. Their &quot;advice&quot; was to manually rebuild the library and use TimeMachine so that there is not only one backup but the history included. I had issues with TM and external drives a couple years ago, thats why I didn't use it. This is one of the unfortunate cases where investigating the issues took so long, it would have been much faster now for me to rebuild it right away.

FYI, I have looked into the following posts:

Photos.app creates blank images after importing iPhoto library
https://www.forbes.com/sites/bradmoon/2019/11/10/how-to-fix-a-damaged-macos-photos-library-when-the-repair-tool-fails/
https://community.spiceworks.com/topic/2227704-apple-photos-library-importing-blank-photos
https://recoverit.wondershare.com/mac-tips/repair-photos-library.html

However, none of these posts helped.
I hate to answer the obvious, but now that you are updated on the OS, I would try once to repair again and open a case with Apple. Worst thing is you spend a little time and someone here has a better answer in the mean time.

https://support.apple.com/en-us/HT204967

In the past there was a repair and a rebuild, but it seems Apple has streamlined their advice before you seek their help. I would hate for you to take more drastic action if there is a bug they have to fix on the rendering and processing side. What if your library is (and was) intact and the code needs fixing?