A cPanel functionality, known as Hotlink protection prevents other websites from directly linking to files on your website. This means that when another website is visited, it cannot load pictures from your pages – this is one of the ways for limiting the outbound traffic for your account.
An example of hotlinking would be a different website using HTML code to display an image from your site. In effect, the other website is using up your bandwidth quota.
Before adding new sub domain, make sure you disable your protection, then enable it back in order to include the new sub domain URL in the include list.
Info : You can check your .htaccess content to make sure the rules are properly aligned to prevent internal server error. This is normally caused by syntax error in .htaccess file.