There is a common fault with OS X 10.10 or higher, where when the user logs in, after a few seconds logs the user logs straight back out again. If this occurs, please follow the steps below:
- Make sure you have all Mac for Office updates and are running the very latest updates
- Go into Settings, Language & Region
- Change the language from "English-Primary" (or whatever the language is set to) to "English-US"
- Go back into Settings, Language & Region
- Change the language back to "English-Primary"
- Open Lync and you should be able to log in just fine this time