Old videos trapped in Photos

I have a number of older videos from 2012 or so, most of which were taken with an iPhone 4 or 4s, that seem to be stuck in Photos.

When I try to open the videos in Photos, no player controls pop up. The videos are .MOV files, and say they are H.264. There is an alert symbol in the corner of the image. Clicking on that alert symbol brings a pop-up saying these are in an unsupported video format.

So, I decided to try to open them with something else. Choosing "Edit with ... Quicktime" brings up an error message that says: "An error occurred while preparing external edit. [File name] couldn't be copied because you don't have permission to access [file name]. You don't have permission." Same message for any other program I try.

Trying to export to my desktop brings a window that says "Export complete with errors. The export operation failed to create a file for the photo identified below." For errors, it says "Unknown error (2)."

Trying to export the unmodified original gives a slightly different error: "[File name] couldn't be copied because you don't have permission to access "Desktop."

Yes, I have checked, and this account does have read/write access for the desktop. Rebooting also did nothing to change the results.

I understand if Photos won't play these videos because of an unsupported format, but it is incredibly frustrating that the videos seem to be stuck in Photos with no way to get them out to try a different program. Any ideas?

FYI 2017 iMac, running Big Sur (11.6).