Summary per Microsoft: Lync for Mac 2011 version 14.0.9 crashes when you try to run it on Apple OS X Yosemite (version 10.10). The program also enters into a sign-in loop if the primary language is set to anything other than English in the Language & Region dialog box.

Direct Link: Lync for Mac 2011 14.0.9 is incompatible with OS X Yosemite 10.10

We will continue to keep you updated once Microsoft has provided a solution.