Lync Mobile 5.4: Can’t verify the certificate from the server.

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.