Please have a look at this forum thread...https://forums.adobe.com/message/7723796
The main message here is "We’ve identified a shift in OS 10.10 behavior that seems to be tripping us up and are testing a fix to roll into our next release. Unfortunately the only workaround until then is to downgrade to the previous version of Lightroom. Thanks to all of you who provided details to help us track this down"
You could try this solution from Victoria:
Go to Lightroom >> Preferences >> Lightroom Mobile
Press the “Delete All Data” button.
From the help menu sign-out of LR
Re-boot your computer
Go to Help >> Sign-in
Try again to sync a collection.