This is an obscure question but I regularly get the error 'unable to sync master' when using CC with Adobe Character Animator. It appears every project contains the 'master' file and some sync and some don't. The other files in the directory sync perfectly (in general).
In the Adobe Character Animator File Structure the 'master' is in this directory.
Character Animator projects are currently incompatible with Dropbox and other file-syncing methods. Do you have one enabled? If so, try turning it off while Ch is running (and don't make any changes to a project on two machines at the same time -- the resulting synced project may not open). Syncing while Ch is not running should be fine.
Perfect - thanks Dave!