The document could not be opened. An unknown error occurred (-12842)

It is a video file, in .mp4 format. It will play in the finder preview window, but a double-click results in the error. I have rebooted (twice), and the file just won't open.

It will open in VLC.

This is a brand new issue - as in NO files with ".mp4" will open. They were all working this morning. No software has been installed or updated (that I am aware of). Alas - something has changed, and I have no idea what.