Hi, thanks for your response. I restored the Photos Library from a backup (changed my Mac several days ago so I basically restored the old Mac to the new) and only today did I get the pop-up (I did open the Photos a couple of times in the meantime). When I realised it was either accept or quit, I accepted to delete the incomplete items, and then followed the instructions:
To download the complete library, delete the incomplete items, then set this
library as the System Photo Library in Preferences and enable iCloud Photos.
This will disable iCloud Photos on any other libraries.
Now I have 960 photos as opposed to 6,000 on iCloud. So you are basically saying that the 5,000 missing photos will sync to the library? Does it make any difference if I choose Download Originals to this Mac or Optimise Mac Storage?
Thanks again!