After changing the hostname of the server, the hostname can no longer be used to access cPanel & WHM services securely with an SSL.
The SSL certificate was installed was on the old hostname and does not include the new hostname.
Option 1: If you have obtained an SSL certificate for the new hostname from a third-party provider, you may use WHM >> Manage Service SSL Certificates to install the SSL certificate.
Option 2: If you are using the free cPanel-signed hostname SSL, run the following script to request a new SSL:
Further information on installing the free cPanel-signed SSL can be found here: Install the free SSL Certificate on the server's hostname