This issue occurs if you have a self-signed certificate installed on your server. If you are sure, you installed your certificate correctly, the reason for the error might be a shared IP address for both of the certificates, if server does not allow using SNI technology. You can find more information on this here. To resolve this issue, you need to remove the self-signed certificate and reinstall the certificate issued by the Certificate Authority once again.
For installation instructions, please refer here.