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
- In photos app, select movie
- File -> Export -> Export Unmodified Original
- Click "Export" button in dialog
- Choose the destination folder
- Click "Export Originals"
- Go to the destination folder in finder
- Open the exported original with Quicktime
- Trim video
- Save
When following the above steps, the save completed successfully.