The "share" function just hangs, and creates a small, incompatible .mov file that Q-Time won't even play.

This title is lifted from a 2021 post. I am using the most current iMovie on my M4 mini. It had been working fine. I was sharing my iMovie projects using the Social Media option and I was getting 1080p quicktime files which I’ve been uploading to my Youtube channel. Out of the blue the Share feature stopped working. I am using a library on a 256 Gb thumb drive and I was having no problem. Now the progess pie shows no action and there is no error message. Where I was expecting a 1.5 Gb quicktime file, I get a 100 Mb unplayable file. I have followed the suiggestions from that 2021 post, including deleting preferences and moving my project to a different location, even on a different Mac. I still cannot share my project. I can only surmise that somehow my project got damaged though I see no evidence of that and there are no messages of any kind. I have no idea how to fix it.