Today, I ran across the issue of some Lync clients not being able to connect up to our Lync 2010 server. They were receiving the following error:
Sign in to Microsoft Lync failed because the service is not available or you may not be connected to the internet.
Turned out this was caused by having an invalid SSL certificate. The reason the certificate was invalid was this particular Mac was not apart of the domain and therefore the root CA's certificate was not trusted. Simply importing the root certificate into the keychain logged the user right in.
Where do you get the root certificate, and how do you import it (mac)?
Your systems administrator should be able to provide the root certificate to you. Unfortunately, I don't have any step by step documentation on how to find the lync server your client connects to, where you could obtain the certificate. I do know though, that if you enable client logging you can find the server your machine connects to and navigate to that server via a webbrowser. From there, you should be able to view the certificate chain, which should let you establish a trust.
Hope this helps,