PHP MySQL server for C# application (406 not acceptable)

I am trying to setup a PHP site to handle the connection to a MySQl database for a C# application. But I can't get the communication between the C# application and the PHP site to work. I always get an "406 Not acceptable" error.

Here is the code from PHP.

<?php
header("Content-type: text/plain");
echo "test";
?>

Here is the C# part.

var request = (HttpWebRequest)WebRequest.Create("http://manoest.dk/UpdateValue.php");
request.Accept = "text/plain";
var test = request.GetResponse();

//WebClient client = new WebClient();
//client.Headers.Add("Accept", "text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,*/*;q=0.8");           
//string test = client.DownloadString("http://manoest.dk/UpdateValue.php");

I am pretty experienced in C#, but completely new to PHP. Hope that you can help me out.