Modulus mismatch, key file does not match certificate
I got this error message when trying to install an SSL certificate in Webhost Manager, the Cpanel hosting administration interface.
This error comes from not having the .key that matches the cert you are trying to install. When you go through the SLL certificate creation process, you have to generate a Certificate Signing Request(CSR). Your Webhost may do this for you, or you might have done it in Cpanel or WHM. In any case, the CSR contains the .key that matches your SSL cert. It's the Private Key.
Copy this and paste it in the box below the SSL certificate.
Note: When you paste in the SSL cert, as soon as you click outside the SSL text entry box, or <Tab>, the server will fill the key box with something. You have to "select all" in the second box, and then paste your key instead.