When installing an SSL certificate, if you get a “Modulus mismatch”, that means that the KEY file does not match the key used to generate the SSL certificate.

Double-check that the key in WHM matches the one on the server.  If the key matches what’s in the files, then the user did not key the SSL correctly.

You can retrieve the KEY file in shell (SSH):

support@somehost [~]# ls -la /etc/ssl/private|grep somedomain.com
-rw-r–r– 1 root root   1675 Feb 11 14:34 www.somedomain.com.key

support@somehost [~]# cat /etc/ssl/private/www.somedomain.com.key
—–BEGIN RSA PRIVATE KEY—–
KEY CONTENT IS HERE…
—–END RSA PRIVATE KEY—–

If the key is different that the one displayed in WHM, then copy it and paste it in the KEY area in WHM.

Make sure that you paste the SSL CRT text first, then proceed with the rest of the information.