This error is returned when a resource that the user is attempting to access requires a valid client Secure Sockets Layer ...

This error is returned when a resource that the user is attempting to access requires a valid client Secure Sockets Layer (SSL) certificate. A client certificate is used for identifying you as a valid user of the resource and must be installed in the Web browser. The error message can also occur if IIS is unable to contact the server that stores the Certificate Revocation List (CRL) to check for revoked certificates. If a CRL server is unreachable, the certificates are presumed to be revoked.