The certificate is not trusted because the issuer certificate is unknown. Error code sec_error_unknown_issuer. So people have mentioned that the intermediate certificate is need but I think that I have installed that as well as the main certificate.
This message seems to appear with Firefox and seems to be ok with the IEs
You didn’t install the certificate chain on your server.
Your cert is signed by “AlphaSSL CA – G2″, which is itself signed by the root CA Cert, which is what browsers trust. Your server has to send the chain so browsers can follow it and figure out the trust.