1) The problem happens when I connect my iPhone 7+ IOS14.1 to my iMac OS Catalina. I have restarted them many times, I have even reinstalled Catalina.
2) Photos works OK on the iMac & ok on the iPhone but not when connected - the iMac starts to prepare photos for import, after a certain number ( +/- 6'000) the thumbnails cease to show up as photos & mostly all become grey.
3) I don't know why photos on the iMac wants to import 10'000+ photos from my iPhone as 99% of them were already imported before I updated to Catalina ! The originals are now mostly duplicated on my iMac & on my iPhone.
4) I cannot open a thumbnail to edit it. I select my iPhone from the sidebar and the only action I can do is click on a greyed out thumbnail to select the (non-visible) photo for importation, which causes a blue tick to appear on the greyed out thumbnail.
5) I spoke to various apple helplines online & the last person I spoke with said it was a recent bug, the engineers were working on it & there would one day be an update ( iPhone or Mac I'm not sure which) that would fix it.
That's the full story, over to you ...