As I mentioned in another thread with you could you please send me a LR Desktop diagnostig log - best as a private message with a downloadable dropbox link.
You can trigger the log via LR Desktop prefereneces -> Lightroom Mobile and when you hold down the alt key you will notice a generate diagnostic log button.
Last night after I posted my question I found another thread where you said to delete sync.lrdata. I did and that fixed my problem.
On Mac it's at /Users/<user>/Library/Caches/Adobe/Lightroom/Sync Data/Sync.lrdata
I spent a lot of time tracking this down. I would like to see information like this made much more accessible from the main help page.
Glad that it's working again.