If an attempt to upload files with the Flash Upload method in any of the Manager plugins fails with a Upload Failed  - 403 : Forbidden, Upload Failed  - 406 : Not Acceptable or an Upload Failed - 503 : Service Unavailable error, this may be necause of security settings on your server preventing the Flash upload script from working properly.

If possible, add the following to your .htaccess file:

<IfModule mod_security.c>
SecFilterEngine Off
SecFilterScanPOST Off
</IfModule>

If you are unable to make this change, you can switch the uploader to 'HTML Mode' in the JCE Configuration (Components->JCE Administration->Configuration). This is a simplified uploader without a progress indicator or the ability to upload multiple files.