The configuration validation is an optional way to have CDN Enabler validate your CDN hostname. All it does is make a test request to an asset hosted on your website with the inputted CDN hostname. The asset being tested is the minified CSS file, where the path is currently wp-content/plugins/cdn-enabler/css/settings.min.css. This can be different depending on your installation, but CDN Enabler will find the correct path and make the aforementioned request.
If successful, meaning a 200 status is received, it will be valid. If unsuccessful, meaning any other status code is received, it will be invalid. This is made available to help prevent CDN Enabler from rewriting assets that would lead to errors and “break” the website. Using felebredvehu.azureedge.net or https://felebredvehu.azureedge.net would result in the same request being made as the hostname being used is felebredvehu.azureedge.net. The reason given afterwards should help you troubleshoot this further.
If you don’t want CDN Enabler to perform this validation it can be bypassed by clicking “Save Changes” instead. This will then save the CDN hostname you’ve inputted and proceed with rewriting your site hostname to the CDN hostname, regardless of whether the assets will be successfully delivered by your CDN.
