After syncing my Photos library to iCloud Photos I was left with 86 photographs which Photos was unable to upload.
It wasn’t initially clear why they were unable to be uploaded.
The solution I found on the web was to export the original files and then delete them from Photos and then import them into Photos.
When I tried that I got a 47,001 unknown error, which wasn’t really very helpful.
However after doing some more searching, the issue appeared to be that the original files were missing from the library and all that remained was the low resolution thumbnail.
As I had already made a back up of my photographs to Amazon Photos, I was able to find the originals on that service, download them, import them to Photos and then upload to iCloud Photos.
All fine now.
Makes you realise the importance of having your photographs in more than one place.
I have a minimum of two backups, one on an external drive and one in the cloud.