1. Open the utility Keychain Access – from Finder to go Applications and then into the Utilities folder. Or just search for it via Spotlight.
2. In Keychain Access – in the left pane click on “login” on the top, and “Certificates” on the bottom.
3. In the right pane, find a certificate with the name “unknown”. Select it and delete it. You may have to unlock your keychain with your password.
4. Close Keychain Access.
5. Restart Microsoft Lync.