Think I found a fix for this solutions. Keep in mind there were other people experiencing this problem that followed other instructions that worked for some but didn't work for many and a lot of message threads with no solution.
Here is what I did to fix Adobe Lightroom CC v2.3... I had 3 Photos that were in a infinite loop "Syncing 3 Photos". Go to https://lightroom.adobe.com to access your Lightroom Web (Sync). In the left panel you will see "Deleted" section. Select that. Now browse for images that don't have thumbnails and delete them completely from your Lightroom Web Catalog. You can also just delete all the images in the Deleted section if your don't care about restoring anything. But in my experience I just deleted the 3 images that didn't have thumbnails to get the results.
That's how I fixed my "Syncing # Photos" problem.
Cheers!