This is more than likely because the intermediate certificates were never installed. Installing them should resolve this error. Below you will find links on where to locate and install your intermediate certificate, depending on the Certificate Authority that issued the cert. Or you can always contact your SSL provider.

Related posts:

My certificate works in my browser, but my visitors get a Security Alert that says ‘The security certificate was issued by a company you have not chosen to trust…’ What is the problem? Where do I get my Intermediate certificate? What is a Certificate Authority and what is your relationship to them? How long are your SSL certificates valid for?