There is a known issue with Lync clients with new Apple OS called Yosemite. The Lync for Microsoft Office 2011 (with Office 365 or Lync on premise) no longer will sign in, the potential fix is listed below.
Primary Symptom: when you attempt to log into Lync, it appears to sign you in just fine, but after briefly displaying your Lync contacts, it immediately signs you out and gives you a generic error message that either the server is unavailable or some unknown error has occured.
- 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