Catalina Photos "The export operation failed to create af file for the photo identified below" "Unknown Error (4)"

Select a single photo in Photos and export it: "File -> Export 1 Photo..." or "File -> Export Unmodified Original for 1 Photo..."

Error message: "The export operation failed to create af file for the photo identified below" "Unknown Error (4)"

For some photos the export works fine 100% of the time

For other photos the export fails with the above error message 100% of the time

I already tried shutting down and restarting my computer.

I used to have "iCloud Photos: Optimize Mac Storage" enabled in the past, and about 2 weeks ago I changed it to "Download Originals to this Mac".

My photo library used to be on a different MacBook, but I replaced the MacBook with a new one and transferred everything from the old MacBook to the new MacBook using a time-machine backup and the migration tool.

I have 121,319 photos in my library, and plenty of free disk space (290 GB of 2000 GB still available)

When the error does occur, the following line always (every single time) appears in system.log:

Jul 20 12:54:53 Brunos-MacBook com.apple.xpc.launchd[1] (com.getdropbox.dropbox.garcon.3AABFC2F-8F72-41F7-BFB3-7BAFB5D370F8[1300]): Caller wanted oneshot behavior for pre-existing instance: caller = com.apple.appki

(Except the UUID is different in each log message)

Photos 5.0 (151.19.150)

macOS Catalina 10.15.5 (19F101)

MacBook Air (Retina, 13-inch, 2020)