OWin post to localhost fails unless Fiddler is running - wpf-controls

Question

I have a self-hosted OWIN application that embeds a System.Windows.Controls.WebBrowser control in a WPF view. This browser connects to a specific site which then communicates to my application via POSTs to 127.0.0.1:8002. The architecture is not under my control so not open to change.
The site is loaded happily and runs fine. The localhost communications only works if I have Fiddler running. If I take out Fiddler, I get an error.
I can do the post directly to the application using PostMan without Fiddler - no problems here.
I'm guessing normally everything is in-process and causing problems but Fiddler forces it to be out-of-process and invokes some marshaling magic that fixes the problem. Just a guess. I've tried running the OWIN service in a different thread; didn't help.
I've seen a similar (working) sample application, but it used Awesomium running in a separate process. This is not an option, we must run a specific version of IE.
Any thoughts on how to get OWIN to talk to the browser control?

Answer 1

The problem was solved by initialising OWIN with a wider range of URLs. Initially, I was just doing this:
StartOptions options = new StartOptions();
options.Urls.Add(String.Format("http://localhost:{0}", port));
WebApp.Start<ServiceStartup>(options);
When I added further URLs, the problem goes away. I'm guessing that Fiddler is changing the URLs on the way through.
StartOptions options = new StartOptions();
options.Urls.Add(String.Format("http://localhost:{0}", port));
options.Urls.Add(String.Format("http://127.0.0.1:{0}", port));
options.Urls.Add(string.Format("http://{0}:{1}", Environment.MachineName, port));
WebApp.Start<ServiceStartup>(options);