@ryanne77: The problem still happens when I boot in safe mode.
@ryanne77: New clue: when I try to edit one of the affected photos in the Photos app on the new MacBook, I get a "Cannot Start Editing: Photos cannot load adjustments for this image" error. Screenshot at https://imgur.com/nOi4a4K
@leonie: The files are JPEG. When I downloaded from iCloud I tried both "unmodified original" and "most compatible": both work.
Detailed current state:
* I have selected 3 Photos that consistently reproduce the issue for the experiments described below. (There are many thousands of other photos that also have the issue.)
* The Photos application on the new MacBook 2020 (Catalina) consistently reports "Unknown Error (4)" when I try to export the photos. It fails for both for normal export and "export unmodified original"
* When I try to edit one of the affected photos in the Photos app on the new MacBook, I get a "Cannot Start Editing: Photos cannot load adjustments for this image" error. Screenshot at https://imgur.com/nOi4a4K
* When I boot my new MacBook 2020 in "Safe Mode", the problem persists. Any attempt to export the photos still results in the same "Unknown Error (4)". I made sure that I actually was in safe mode: there was a red "Safe Boot" message in the top right of my screen.
* Possibly unrelated to the issue at hand: while in safe mode, I did notice that my MacBook was noticeably slower. All animations were stuttering. Repainting of the screen was slow (about 50ms per repaint) and I could clearly see the screen being repainted in horizontal bands.
* The Photos application on the old MacBook 2016 (Mojave) with the same Photos library (before it was migrated to the new MacBook and still synced to the same iCloud) consistently exports the photos successfully. It succeeds for both for normal export and "export unmodified original"
* The iCloud web-based photos application (using a browser) consistently downloads the photos successfully. It succeeds for both "download unmodified original" and "download mosts compatible including edits" in the "secret" drop-down menu.
* However, the first time (and only the first time) I tried to open the album in the iCloud web-based photos application I got an error message "Photos has stopped responding. An error has prevented this application from working properly." I selected "Send to Apple". This happened 6-Aug-2020 9:05am Netherlands time. I was using a Chrome 84.0.4147.105 browser. A screenshot is here: https://imgur.com/UkJC88g
* All 3 photos were taken and added to my library in late 2016 or early 2017
* All 3 photos are JPEG format (file extension JPG).
* Since these are old photos, it is quite possible that I used a (much) older version of Photos, or iPhotos, or Aperture to edit them in the past.
* For one of the photos I did a "Revert to Original" and then tried to export again. The revert operation worked, but it did not help: subsequent export attempts still failed with the same "Unknown Error (4)" on the new MacBook.
* Also likely unrelated to the issue at hand: yesterday I enhanced a few hundred photos trying to reproduce another issue (namely https://discussions.apple.com/thread/251652645). Since then (until I rebooted my MacBook in safe mode today) the photolibraryd daemon was pegged at 100% continuously.
