URLRequest with crossdomain behind firewall gets 2044/2048 Sandbox violation

Hi!

We have an app running over HTTP on server "S", behind a firewall "F" and requesting an XML file through HTTPS to server "X" (through the firewall).

We've setup the crossdomain policy files on "X" as follow :
https://domain_X/crossdomain.xml

<cross-domain-policy>    <site-control permitted-cross-domain-policies="all" /></cross-domain-policy>

https://domain_X/crossdomain_for_firewall_F.xml

<cross-domain-policy>    <allow-access-from domain="ip.of.firewall.F" secure="false" />    <allow-http-request-headers-from domain="ip.of.firewall.F" headers="*" secure="false" /></cross-domain-policy>

secure="false" because the request comes from a non HTTPS page.

The URL we're trying is accessible without any problem outside of flash using its URL:
https://domain_X/some_folder/file.xml

In our flash souce code we call

Security.loadPolicyFile(https://domain_X/crossdomain_for_firewall_F.xml) 

and it seems to be loaded properly as reported by Fiddler2

Then we use the URLRequest to get the xmlfile as follow:

var request : URLRequest = new URLRequest (https://domain_X/some_folder/file.xml);
request.method = URLRequestMethod.GET;
var loader : URLLoader = new URLLoader ();
loader.addEventListener( Event.COMPLETE, getfeatureinfo_result )
loader.load ( request );

All of this works fine when running under flashbuilder as it doesn't request the crossdomain policy file. But when it get published on our test server, we get the 2044/2048 Sandbox violation error.

We use IE8

I really don't know where to look next to find a solution. I guess it has something to do with the firewall?

Any suggestion would be greatly appreciated.