QuickTime Player can’t open movie - The file isn’t compatible with QuickTime Player.

A follow up to QuickTime Player can’t open movie - The f… - Apple Community

but silly apple discussions always locks down threads and prevents useful followups after a few days.

When trying to trim a video, opening it from the photos app, upon trying to save, the error message in the title is displayed and the file does not save.

I worked around this by

1. In photos app, select movie
2. File -> Export -> Export Unmodified Original
3. Click "Export" button in dialog
4. Choose the destination folder
5. Click "Export Originals"
6. Go to the destination folder in finder
7. Open the exported original with Quicktime
8. Trim video
9. Save

When following the above steps, the save completed successfully.