Common reasons for this error are:
* Incorrect file/directory permissions: Above 755.
In order files to be processed by the webserver, their permissions have to be equal or below 755. You can update file permissions with a FTP client or through cPanel's File Manager.
* Incorrect Apache directives inside .htaccess file.
Make sure you have not specified unsupported directives inside the local .htaccess file. Such include PHP settings and Apache module settings.