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.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section. We plan to release a new version of Lync for Mac 2011 (as version 14.0.10) that will provide full support for OS X Yosemite 10.10.
The third-party products that this article discusses are manufactured by companies that are independent of Microsoft. Microsoft makes no warranty, implied or otherwise, about the performance or reliability of these products.