Request handled okay when issued by browser, but not when issued by ASP.NET application on the same machine

I'm debugging two ASP.NET applications running on the same machine under different instances of Cassini and with "custom errors" off. One app is running on port 100 and wants to perform a GET request from the other app running on port 90. So it runs this code:

WebRequest request = WebRequest.Create(
   "http://localhost:90/Controller/Action?Param1=foo&Param2=bar");
request.Timeout = 10000;
request.GetResponse();

and the last line throws a WebException with HTTP 400 code and null InnerException. If I copy the very same URL in clipboard, past it into IE running on the same machine - the request is queued to the app on port 90 and its /Controller/Action/ is invoked and even parameters are passed okay.

What could be the problem origin here and how do I solve it?

Two hours debugging - and it turned out that service at port 90 would redirect the request back to the service at port 100 but wouldn't provide a required parameter in the URL, so the handler in the service at port 100 would throw an exception and return the HTTP 400 which was then reported by the GetResponse(). The solution was to change the logic so that there's no redirect for this specific request because the redirect would make no sense for this specific request.

And the jury finds both Cassini and ASP.NET to be not guilty.