Problems with .mov videos in Mac Photos, Sequoia 15.5 on a 2012 15" 2.6GHz quad core

Hi all. I've got a great machine here and have found very few problems so far. But I can no longer edit or export video files from the Photos app. I record videos with a canon powershot or an iphone. They both import (slowly) into Photos app on my mac and appear as .mov files. Basic edits are not available. When shortening a video I can draw the cursors in to shave off the beginning or the end, but there is no option to Save, so it reverts. When I export at a lower resolution it stalls for 5 minutes and then says it failed. Sometimes the exported video appears later on the desktop, but at full resolution again. When I upload them to youtube, I get a notification that upload has finished, but "Processing failed".

Anyone else having these problems?

I can still upload the same video directly from iphone to youtube, so there is nothing wrong with the original .mov file, or with my youtube account, or wifi/upload, etc.

Author: @Roger_Ales