URLLoader doesn’t even try to load. Error #2032: Stream Error

I am trying to use URLLoader to load an XML file from server (same domain as swf). This should be too simple but I am getting Error #2032: Stream Error

If I trace the HTTP status event it just shows status=0 though I have read that Mozilla doesn't supply status codes to Flash Player so maybe it's not informative.

I have sniffed the HTTP traffic with Charles and Flash isn't even trying to load the url – no request is made, so it doesn't even fail.

I can browse to the url, which is on an internal url that looks like: http://media.project:8080/audio/playlist.xml

I have tried putting a crossdomain.xml in there (with and without a to-ports="8080"), though it shouldn't need one.

Neither the onOpen nor onActivate events fire, just one HTTPStatus and then the IOError.

I have copied the common URLLoader code from Adobe example, mine looks like this:

    public class PlaylistLoader extends EventDispatcher
{
    public var xmlLoader:URLLoader;
    public var url:String = '';

    public function PlaylistLoader(url:String)
    {
        url = url;
        trace(url);
        xmlLoader = new URLLoader();
        xmlLoader.addEventListener(IOErrorEvent.IO_ERROR, onIOError);
        xmlLoader.addEventListener(Event.COMPLETE, onResult);
        xmlLoader.addEventListener(HTTPStatusEvent.HTTP_STATUS, httpStatusHandler);
        xmlLoader.addEventListener(Event.OPEN, onOpen);
        xmlLoader.addEventListener(Event.ACTIVATE, onActivate);
    }

    public function loadData():void {
        var req:URLRequest = new URLRequest(url);
        trace(req);
        xmlLoader.load(req);
    }

    protected function onResult(e:Event):void
    {
        var xmlData:XML = e.target.data as XML;
        parseData(xmlData);
    }

    private function httpStatusHandler(event:HTTPStatusEvent):void {
        trace("httpStatusHandler: " + event);
    }

    protected function onOpen(e:Event):void
    {
        trace(e);
    }

    protected function onActivate(e:Event):void
    {
        trace(e);
    }

    protected function onIOError(e:IOErrorEvent):void
    {
        trace(e);
    }