http://blogs.msdn.com/b/jpsanders/archive/2007/10/17/400-error-from-webservice-using-net-2-0-and-preauthenticate.aspxPrashant from my team found this interesting issue. I will blog it here until the KB comes out on this. Problem ====== WebService server responds back with a HTTP 400 error when using PreAuthenticate on the WebServiceProxy class. This results in aen-USTelligent Evolution Platform Developer Build (Build: 5.6.50428.7875)http://blogs.msdn.com/b/jpsanders/archive/2007/10/17/400-error-from-webservice-using-net-2-0-and-preauthenticate.aspx#8554828Tue, 27 May 2008 16:14:44 GMT91d46819-8472-40ad-a661-2c78acb4018c:8554828Jeff Sanders<p>I would suggest you contact Microsoft Support. &nbsp;<a rel="nofollow" target="_new" href="http://support.microsoft.com">http://support.microsoft.com</a></p> <div style="clear:both;"></div><img src="http://blogs.msdn.com/aggbug.aspx?PostID=8554828" width="1" height="1">http://blogs.msdn.com/b/jpsanders/archive/2007/10/17/400-error-from-webservice-using-net-2-0-and-preauthenticate.aspx#8554250Tue, 27 May 2008 10:46:53 GMT91d46819-8472-40ad-a661-2c78acb4018c:8554250bivarsoy<p>Hi. When you write Proxy do you mean an ISA Server? </p> <p>We experience this problem in a similar context:</p> <p>We have a Win XP .NET 2.0 GUI appliction using web service proxies communicating with integrated windows security towards Web services (proxy services) running on a W2K3 &nbsp;which again flows through an ISA Server towards actual back-end Web services (here we use anonymous access). This fails with or without setting PreAuthenticate to false or true with HTTP 400 - &quot;server cannon clear headers&quot; message. &nbsp;The client has the basic .NET 2.0 run time redistributable while the servers have .NET 2.0 with service pack 1. </p> <p>Any suggestions?</p> <p>Tx. Bjorn</p> <div style="clear:both;"></div><img src="http://blogs.msdn.com/aggbug.aspx?PostID=8554250" width="1" height="1">http://blogs.msdn.com/b/jpsanders/archive/2007/10/17/400-error-from-webservice-using-net-2-0-and-preauthenticate.aspx#5499424Thu, 18 Oct 2007 00:15:39 GMT91d46819-8472-40ad-a661-2c78acb4018c:5499424Computers » 400 Error from WebService using .NET 2.0 and PreAuthenticate<p>PingBack from <a rel="nofollow" target="_new" href="http://www.soundpages.net/computers/?p=2708">http://www.soundpages.net/computers/?p=2708</a></p> <img src="http://blogs.msdn.com/aggbug.aspx?PostID=5499424" width="1" height="1">