“cannot open” when trying to save screen recording in quicktime player
Sounds like a permissions problem. If it’s a bug I suspect it will be fixed in a future macOS update. Not much help, I know.

Until then, you have to peek behind the curtain:


Unsaved recordings are stored at ~/Library/Containers/com.apple.QuickTimePlayerX/Data/Library/Autosave Information/
The file will be called Unsaved QuickTime Player Document.qtpxcomposition, or similar
Show Package Contents on that file
Inside is Screen Recording.mov which you can copy and keep