Have you tried to re-sign-in via the Creative Cloud app and restart Lr Desktop? - Guido
Now I was able to get it work.
When I updated the CC lightroom at first time, the Mobile CC sync did not work, but after uninstalling and the updating it, the sign in for mobile cc appeared in the lightroom cc and the mobile sync started working.
Dont know what was the problem, but now all my photos are synced also in mobile cc folder.
One bug in lightroom is that you cannot change the mobile cc sync folder to a another hard drive and it uses the default drive (C:) as a saving location.
I hope, the bug is already at the repair list..
I am having a similar problem -- I tried to sign in to Lightroom Mobile. I entered my Adobe ID and password and the dialog went away, but I still see "Get started with Lightroom mobile" and when I select Sync with Lightroom Mobile, the dialog box appears briefly and then goes away.
I have tried signing out of Adobe Creative cloud, signing back in, and re-launching Lightroom, but that did not solve the problem.
Do I have to uninstall and reinstall Lightroom?