This error will occur when your webserver, firewall or network has not been correctly configured to serve pages over SSL.
Check the following:
- Your certificate has been installed for the correct website
- Your private key is not corrupt or has not been accidentally deleted
- You have assigned port 443 as the SSL port on your webserver
- You have opened port 443 for SSL traffic on your firewall or router
- You have correctly configured your DNS settings on your network