If you see an error that "The site can't provide a secure connection" or ERR_SSL_PROTOCOL_ERROR, this is due to the webserver not providing an SSL certificate.
The error message may look like:
You may see this error if you have enabled a site through CloudFlare, but the SSL certificate isn't available. You can test this via an OpenSSL command:
openssl s_client -showcerts -connect www.yourdomain.com.au:443
If you see a copy of the certificate, as well as details of the issuer. For example, this is from a working system:
Server certificate subject=/CN=www.yourdomain.com.au issuer=/C=US/O=GeoTrust Inc./CN=RapidSSL SHA256 CA
If you don't have a valid certificate, you'll see something like this:
SSL handshake has read 7 bytes and written 305 bytes --- New, (NONE), Cipher is (NONE)
This means that no certificate was available and therefore the OpenSSL client was unable to establish an encrypted connection.
To resolve, you'll need to ensure that your webserver or Content Distribution Network (CDN) provider has properly enabled the SSL certificate for your website.