Could you try to trigger a "Delete all Data" via Lightroom Desktop preferences -> Lightroom Mobile. With that you would start from zero.
When you still come to the point that sync hangs 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
Thanks,
Guido