This is caused by either a permissions issue, or a lack of index.php or index.html file in the document root (public_html unless using add on domains) of the domain in question.
Either:
1) Create or upload the relevant index.php or index.html file to the document root of your domain.
2) Contact us should you believe there's a permission issue in effect.
3) If you would like files (instead of a web page) listed when someone access your site, add this to the top of the .htaccess file in your domain's document root (you may need to create this file):
Options +Indexes
