To correct the issue, disable any security plugins and check to see if the problem is resolved. Once you determine that a plugin is causing it, you will then need to contact the plugin developer for assistance in isolating the setting that is blocking it.
The following plugin settings are known to cause this problem:
This block can also occur due to an htaccess setting such as this one that is created by the WP Defender plugin.
WP Defender - Protect PHP Executed ##
Deny from all
deny from all