Photos upload from Mac to icloud stuck

Photos upload to icloud stuck - showing error:

16/03/2018 5:54:00.765 PM com.apple.photos.VideoConversionService[847]: [17:54:00.765] VTCompressionSessionSetProperty signalled err=-12900 (kVTPropertyNotSupportedErr) (Unsupported property key (it's not in the SupportedPropertyDictionary)) at /Library/Caches/com.apple.xbs/Sources/CoreMedia_frameworks/CoreMedia-1731.15.20 7/Sources/VideoToolbox/VTCompressionSession.c line 1883

I had pretty much completed the upload of around 23k items and then it seemed to start again only this time the number of uploads it says are needed is bigger than the number in the library. When I select 'pause for a day' it then shows closer to 4k photos to upload.

Any help much appreciated as I'm completely lost!

Thank you.