Could you try a delete data. Quit Lr desktop, and the delete the sync.lrdata file.
You can find the sync.lrdata file here:
On Mac it's at /Users/<user>/Library/Caches/Adobe/Lightroom/Sync Data/Sync.lrdata
On Win it would be at C:\Users\<user>\AppData\Local\Adobe\Lightroom\Caches\Sync Data\Sync.lrdata
Restart Lr desktop and enable the collections for sync but keep the "iPad Photos" collection disabled for now.
In case the collections-sync successfully finished, trigger a new import on your iPad to have your iPhone photos synced as well.
you add new photos to your mobile device, Lr mobile needs to import the
photos locally and send previews+metadata to the back-end. It could be
that some of this data was not send and due to this Lr desktop sync
started to hang.
let me know how it's working
thanks, that worked after the restart!
You are Welcome