I'm sharing this to also hopefully help fellow users who ran in to the same problem as me today. With the release of Apple's latest operating system OS X 10.10 Yosemite, I found that everything worked well apart from Microsoft Lync 2011 for Mac.

The problem with Lync 2011 for Mac:

Attempting to sign in was quickly followed by Lync signing out again with an error saying there no internet connection - when there was.

So I tried some things; making sure Office was all updated, cleared even uninstalled and reinstalled Lync completely but this failed to fix the issue. What did work was completely random and by sharing this I hope to help other resolve this bug too.

  1. Open up System Preferences on your Mac
  2. Go to 'Language and Region'
  3. Change the language to English - US
  4. Restart your Mac - Make sure to press the close button in the top left of System Preferences
  5. Lync 2011 Mac issue with OS X 10.10 Yosemite

  6. Choose Restart
  7. Once your Mac is back up and running, return to System Preferences and return your language preferences to what they were before
  8. Reboot again using the same method as explained in step 4.
  9. Run Lync again and it should sign in this time

After doing this I hope it will get you back up and running with Lync on your shiny new Yosemite Mac like it did for me. Now go and enjoy Yosemite, it's awesome!