checkPolicyFile flag was not set
Sometimes the uploading of crossdomain.xml file is not enough to get the things working.
I just released a site for my client. I uploaded all the stuff on the server including crossdomain.xml file, but the site didn't works with "www". The error that popups was:
A policy file is required, but the checkPolicyFile flag was not set when this media was loaded
So the solution is pass a LoaderContext to your Loader object:
var context: LoaderContext = new LoaderContext();
context.checkPolicyFile = true;
var loader: Loader = new Loader();
var img_url: String = img_src;
var request_url: URLRequest = new URLRequest(img_url);
loader.load(request_url, context);