SSL cert purchase and installation for Cpanel

The modulus is the cryptographic algorithm used to create the public/private keypair.  If these do not match then either it got corrupted somehow, or more likely you might have generated another CSR (Certificate Signing Request) or removed the original CSR prior to installing the commercial cert.

When you issue a cert, you do it for the server - when you created it may have just been issued to the server name.  It should have been issued to the server's FQDN instead (www.yourdomain.com).  If this was not done, you would need to get another cert with the correct name.  To double check, open up the cert (just double click it) and look at the general tab - if this matches, great, if not then you might also check the Details tab for Subject Alternate Name to see if there might be other names listed there that might match properly.

If you do need to reissue, don't worry.  Most commercial CA's understand that problems happen, just call them up or email them and explain the situation and they will normally work with you to get a new CSR created properly and issue a new cert for free (since you already paid for it before - they would just revoke the bad cert).  I haven't heard of one yet that doesn't not operate this way.

http://www.cpanel.net/support/docs/11/cpanel/sec_ssl.html
http://www.cpanel.net/support/docs/11//whm/ssl_tls_install_domain.html

You can also try to delete the SSL Host entry in WHM/cPanel by hitting the delete button under the domain ( Main >> SSL/TLS >> Manage SSL Hosts ).