Have you tried logging out then back into your Creative Cloud account from Help menu?
Thanks a lot for your help. I just tried to do it, and it is the same issue. The lightroom mobile box doesn't come up when I click on sign in after logging back...
still the same version, and still the same issue. I am a bit lost on this! Just to add some clues, you will find here under information about this.
Sync is disabled for all Lightroom versions <= CC 2015.6.1
For sync to work, upgrade Lightroom to the latest version (current version is 2015.12)
I will reach out to the customer support team to help you in updating Lightroom.
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Please sign out of creative cloud desktop app and rename the caps folder located in the below location:
Sign in to Creative cloud desktop app again and click on install next to Lightroom.
Launch Lightroom click on help and choose system info and check the Lightroom version. It should be 2015.12