So far replaced Lync Client. That did not resolve the issue. Noticed an update to Lync, so we're going to grab that and install. But I'm not hopeful on the reinstall of Lync solving the problem.
Neo, for your #5 suggestion, are you saying to remove all of Office 2011 (all apps) and reinstall? Or, just Outlook? Also, what program installs that button? If someone had Outlook but did not install Lync, would that button still appear (I'm guessing not)?
btw, 14 people use MAC's and they all stopped working about the same time (this button worked fine for 2-3 months). One thing I have noticed is that all MAC Lync users had to use static URI's to get Lync to connect. Do you think their URI is stale and causing button problems? Lync still works, it's just this button within Outlook (Online Meetings) that does not work.