the ssl_error_bad_cert_domain error is shown when the domain name on the certificate doesn’t match the domain of your site. A lot of hosting providers will have a default SSL certificate installed using their own domain. When visiting a site that doesn’t have it’s own certificate installed this error occurs. To test this you can do a test over at the Qualys SSL Labs to check your certificate. If the domain name does not match the SSL Labs test will show it. This can be fixed by installing a certificate for your domain.
This error can also occur when you have configured a certificate with a www or non-www domain while your website uses the other. For example, if your WordPress site address is set to https://www.yoursite.com but the certificate is for the non-www domain (example.com) this will lead to an error. To fix this, add both domains to the certificate like this: