Lync 2011 Sign-in Issues
I just downloaded Lync from Office 365 and had the exact same problem. After a bit of googling I realised that my Lync version was 14.01 and there was an update to 14.09 available. Updating solved my log in problem. Very simple (And stupid: 1. Why does Microsoft not update the package at Office 365? 2. How can login be broken to start with?).