There is a group of recent problems with Photos that, in my opinion, originated from the same source problem. I was working with a local library not connected to iCloud. I have encountered "'Error downloading slideshow photos" followed by "Internet Connection required" when tried creating a slideshow from an album. I did elimination search, adding each time a small group of photos, until I found the photo that causes the problem. I tried to edit the photo and encountered an error "cannot Start Editing. Photos cannot load adjustments for this image". Tried to export the photo and got "Unknown Error(4)". Stayed in editing mode, using the arrow to move quickly to the next photo and found several others with the sam error.
Searched here and found a thread https://discussions.apple.com/thread/250771558 without solution.
So, the bottom line:
There is a bug in Catalina which makes some of the modifications done on photos in the past unrecognized by photos. Since a display version or exported version of a photo is created by taking the master and apply the editing modification on it, such photos cause all sorts of problems. I will report this but to Apple.
