The default HTTP domain validation method requires public HTTP access to your sites, so that means TCP port 80 has to be open and http requests have to reach your server. With the webroot method your own web server has to respond with the challenge response file, so that also means it needs to be listening for http on TCP port 80.
If you update your server configuration to allow http requests for your site the validation will probably work as long as the path you are providing is correct.