I've read all the community posts regarding sync issues and performed ALL solutions, nothing has worked. Note, I understand that you can view the pending images that are syncing in LR, however none are shown.
I was able to recreate a continuous sync issue described by some and kind of resolved one of my issues--what I think could be an underlying issue/bug? The continuous sync error, in my case, I duplicated a photo that has a custom preset applied. I then applied a new custom preset to the duplicated photo. That duplicated photo, ends up having sync issues--sync in a continuous/looping. It wasn't until I removed that photo, that the issue resolved itself. The ironic part of this all is that my mobile devices (ie. iPad, iPhone) seem to be fine or possibly ignoring the issue---all my images are synced, but on my laptop running LR CC and not LrC. I get the continuous sync error.
Note, my sync error is somewhat isolated. I do know when it was last fully synced. I've backed it up and removed the photos before the error started, but no luck.