As the screenshot shows above, 2 photos show as blank in my Lightroom CC, Windows version, up to date. Even I tried to delete it for a few times, they are still there like this. And I think the whole sync is stucked by them.
Thanks for reaching out. We are sorry for the trouble with Adobe Lightroom's Sync. We are here to help!
Could you please confirm if you recently added a large number of images into Lightroom? What kind of files are you working with JPEG or RAW photos and videos? Have you tried changing the internet connection type?
Please log on to Lightroom Web (https://lightroom.adobe.com) to check any current Sync issues. If there are any sync issues, please click on them to know more, find the original for those images, and back them up safely before you empty the Sync issues from Lightroom Web.
Please let us know if this helps! Regards, Sameer K