Hi Devine,
Welcome back. I’d like to collect the following information to do further troubleshooting.
1.Which version of Mac OS are you using?
2.Have you upgraded your Mac OS recently?
To narrow down this issue, let’s try the following steps:
1.Install the latest Office 2011 updates, please refer to the following articles: http://www.microsoft.com/mac/downloads
2.If the updates has been installed, and the issue persists, delete the certificate from Keychain, then restart Lync again.
Thanks,
Vivian Wang