Saving to server not working...

Thank you! It turned out to be CORS and another setting I had to change to allow ASP.NET to accept the request format. For other .NET developers I'll share the web.conf changes I had to set in order to enable this. Now I have to figure out how to decompress the GZIP string
 
<configuration>
  <system.web>
    <customErrors mode="Off"/>
    <compilation debug="true" targetFramework="4.0"/>
    <!-- requires because the Microsoft's request objects don't play friendly http post with header sequences they don't reconize -->
    <httpRuntime requestValidationMode="2.0"/>
    <!-- validateRequest="false" also added for this reason -->
    <pages validateRequest="false" controlRenderingCompatibilityVersion="3.5" clientIDMode="AutoID"/>
  </system.web>
  <system.webServer>
    <httpProtocol>
      <customHeaders>
        <!-- this enables CORS -->
        <add name="Access-Control-Allow-Origin" value="*" />
      </customHeaders>
    </httpProtocol>
    <httpErrors errorMode="Detailed"/>
  </system.webServer>
</configuration>

Thank you ira!
Also posting the C# ASP.NET source to handle the capture. I took this approach because I'm embedding in a WebForms project, but I hope someone finds this useful.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.IO;
using System.IO.Compression;
using System.Text;

namespace FlexDemo
{
    public partial class CaptureDoc : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {
             
            try
            {
                int lineCount = 0;
                StringBuilder sb = new StringBuilder();

          
                using (StreamReader sr = new StreamReader(Request.InputStream))
                {
                    string line;
                    // Read and display lines from the file until the end of
                    // the file is reached.
                    while ((line = sr.ReadLine()) != null)
                    {
                        sb.Append(line);
                        lineCount++;
                    }
                }

                string xml = sb.ToString();
                string dxml = Server.UrlDecode(xml);

                // remove unwanted name-value pairs
                dxml = dxml.Replace("name=myreport.xml&report=", "");
                dxml = dxml.Replace("&type=xml","");
                
                // write report to hard drive
                System.IO.File.WriteAllText(@"c:\temp2\flex.xml", dxml);

                Response.Clear();
                Response.Write("<response><status>Success</status><message>Wrote " + lineCount.ToString() + " lines</message></response>");
                Response.End();
            }
            catch (Exception exc)
            {

                Response.Clear();
                Response.Write("</response><status>Fail</status><exception>" + exc.Message.ToString() + "</exception></response>");
                Response.End();
            }
             
        }
    }
}