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.
- Open up System Preferences on your Mac
- Go to 'Language and Region'
- Change the language to English - US
- Restart your Mac - Make sure to press the close button in the top left of System Preferences
- Choose Restart
- Once your Mac is back up and running, return to System Preferences and return your language preferences to what they were before
- Reboot again using the same method as explained in step 4.
- 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!