If you were brave enough to upgrade your Mac to OSX Yosemite (beta) and your Lync for 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. 

Likely Resolution:

  • Make sure you have all Mac for Office updates and are running the very latest updates
  • Go into Settings (System Preferences), Language & Region
  • Change the language from "English-Primary" (or whatever the language is set to) to "English-US" 
  • Reboot
  • Go back into Settings (System Preferences), Language & Region
  • Change the language back to "English-Primary"
  • Reboot
  • Open Lync and you should be able to log in just fine this time
UPDATE!!!
Thanks to Tiago for updating me, the fix for this issue is being released by Microsoft today; see the official Support KB below related to the known issue: 
Hopefully this will resolve your issue once and for all, as we all eagerly await Mac for Office '16 :-) 
RESOLVED!!
Thanks to the Lync team for providing us with a fix for this issue...see the KB article and download the update now!