This happens if you have an in-house CA and generate and install a certificate in IIS but do not install the associated root certificate. To fix, open Internet Services Manager, select the instance in question and open its properties. Select the Directory Security tab. Click View Certificate. The certificate you have installed will have a red error "x" over it, and an error will be displayed that the sertificate cannot be verified up to a trusted root. Select the certification path, click the root certificate and click the View Certificate button. Click on the Install Certificate butoon. This launches the Certificate Import Wizard. Follow the prompts taking the defaults.
From a newsgroup post: "You might be missing the Intermediate certificate for the server
authentication certificate. Find out who issued you the cert and install their root and intermediate cert on the server."