SSL clients validate the server name against the server's certificate. They download the certificate from the server and make sure the 'common name' attribute of the certificate matches the domain name of the server (FQDN).
If they don't match, clients may flag up an error to tell the client that the server may not be the server they think.
If they don't match, you have to change the common name in the certificate or the FQDN. To change the common name, you'll need to get a new SSL certificate with the correct common name. The only other option is to change the hostname to match the existing common name (if that is possible).
