Your SSL certificate likely doesn't match your custom URL.
Fix:
1) Ensure the SSL certificate covers your custom domain.
2) If using a proxy, configure it to pass the correct SSL.
3) Check for mixed content (HTTP elements on an HTTPS site).
4) Try reissuing the certificate with the correct domain or a wildcard SSL.
To understand in detail how to solve the error, you can refer at CheapSSLWEB.com.