Bad Gateway 502 error when attempting to send XML using C#

I'm having an issue with a payment gateway (Realex RealVault) and neither Realex or myself can work out what the issue is so I thought I'd put this on here to see if anyone could help shed some light on this weird issue.

Everytime I try to send to Realex on the link: https://epage.payandshop.com/epage-remote-plugins.cgi I get an error saying:

502 - Web server received an invalid response while acting as a gateway or proxy server. There is a problem with the page you are looking for, and it cannot be displayed. When the Web server (while acting as a gateway or proxy) contacted the upstream content server, it received an invalid response from the content server.

I am sending my request using the following code (stripped out the debugging code I have in here to return the 502 error for brevity):

HttpWebRequest req = (HttpWebRequest)WebRequest.Create(LinkToRealex);
        req.ContentType = "text/xml";
        req.Method = "POST";
        req.UserAgent = "Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1)";

        req.Proxy = WebRequest.DefaultWebProxy;
        req.Timeout = 15000;

        byte[] bytes = Encoding.ASCII.GetBytes(XMLTosend);
        req.ContentLength = bytes.Length;

        Stream os = req.GetRequestStream();
        os.Write(bytes, 0, bytes.Length);
        os.Close();

        HttpWebResponse resp = (HttpWebResponse)req.GetResponse();
        if (resp == null) return null;
        StreamReader sr = new StreamReader(resp.GetResponseStream());
        return sr.ReadToEnd().Trim();

I have checked IIS logs and everything looks ok there -

2011-01-06 12:44:04 W3SVC636984653 IP_Of_Server POST /RealexTest.aspx - 80 - My_IP_Address Mozilla/5.0+(Windows;+U;+Windows+NT+6.0;+en-GB;+rv:1.9.2.13)+Gecko/20101203+Firefox/3.6.13+GTB7.1+(+.NET+CLR+3.5.30729;+.NET4.0E) 200 0 0

The weird thing is that if I try to send to Realex's other payment system (it uses a different link) everything is fine (by which I mean I get a response back from their server unfortunately I can't use this service for their RealVault service). At first this lead me to believe that everything was down to the URL I was using but because I can put this URL into the browser and get a response I think the issue might be down to IIS or my code.

I'm running asp.net 3.5 code on IIS6/Win Server 05. In IIS the website has an SSL and static IP addr开发者_开发百科ess.

I know this is a bit of a long shot but if anyone has any ideas I could try that would be greatly appreciated.

Thanks for your help, Rich

It turns out that this error is what you see if your XML is not formatted correctly when sending to Realex.

Unlike the normal Realex XML call, which informs you that your XML is not correctly setup, the RealVault XML system will not tell you this and you will see the error message I mentioned in my question.

Hopefully this will help someone else out there.