We’ve been seeing this quite a bit in the TechNet forums recently, users update their iPhone/iPad client and suddenly, we get the dreaded:
“Can’t verify the certificate from the server. Please contact your support team.”
It all worked fine before, nothing has changed with your Lync deployment but all of a sudden the new client won’t log in. Must be a app bug right? Not really, the newest version just has a more strict certificate checks. Since you’re a Lync administrator, you know how serious Lync takes those certificates already. The difference here is a more strict root/subordinate check. Check to make sure these are properly deployed using your certificate vendor’s installation checker (or in a pinch, a cert checker from another certificate vendor may work).
The following KB article outlines it a bit and gives links directly to certificate vendor’s installation verification tools.