Self-hosted OWIN project; http://localhost:8080 won't work, http://+:8080 does?

Question

I have a self-hosted OWIN project that I'm trying to run. It is configured to listen on:

http://localhost:8080

Whenever I try to access that URL I get an HTTP Error 503. The service is unavailable error.

I have run netstat -a -b and made absolutely sure there's no other application running on port 8080.

When I change the configuration to http://+:8080 the project works fine. So the only change is localhost to +.

Now here's the weird things:

• I made sure all entries (localhost:8080 or +:8080) in the HTTP namespace are identical (See this and this, where "iedereen" is dutch for "everyone" (and, yes, this is only on my test-station)). Relevant output of netsh http show urlacl:

  URL Reservations: 
  
  Reserved URL            : http://+:8080/ 
      User: \Everyone
          Listen: Yes
          Delegate: Yes
          SDDL: D:(A;;GAGXGWGR;;;WD) 
  
  Reserved URL            : http://localhost:8080/ 
      User: \Everyone
          Listen: Yes
          Delegate: Yes
          SDDL: D:(A;;GAGXGWGR;;;WD) 
  

• I have the same issue on port 8081 or other ports; "+" works, "localhost" doesn't. So to make sure there wasn't something conflicting with 8080 I've tried 8081 (and other ports, and, ofcourse, with the correct urlacl's)

• When I use +:8080 the application can be reached fine; I get the desired response. So it's not a firewall issue. Nor is it for port 8081 or other ports I've tried. Also, the 503 comes from the application (it is an application error after all, but also: when I stop the application I simply get no response instead of a 503), so, again, no firewall issue.
• I tried running as administrator to no avail, same for running VS as administrator.
• localhost:8080 works on a coworker's computer (which is how this came up in the first place)
• localhost resolves fine to 127.0.0.1 and ::1. No altered hosts file.

I can't seem to debug this issue, I can set a breakpoint in my controller method but it is never reached. The application starts fine.

Ok, to the test-code to reproduce:

using Microsoft.Owin.Hosting;
using Owin;
using System;

namespace TestApp
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            using (WebApp.Start<WebAppStartup>("http://localhost:8080"))
            {
                Console.WriteLine("Press [enter] to quit...");
                Console.ReadLine();
            }
        }
    }

    internal class WebAppStartup
    {
        public void Configuration(IAppBuilder app)
        {
            app.Run(context =>
            {
                context.Response.ContentType = "text/plain";
                return context.Response.WriteAsync("Hello, world.");
            });
        }
    }
}

Again, the above doesn't work unless I change localhost to +.

My main question is: is there a way I can debug this? I get no exceptions on starting the application but the controller method is never reached (or: in the above code, the "Hello, world" is never returned). I've checked the windows eventlog but nothing there.

Note: My problem is not actually a problem (since I have a 'workaround'); when I simply change the config to use http://+:8080 but at this point it's a matter of principle and I'd like to know why + works and localhost doesn't.

I am mostly interested in how to debug this and what the difference between localhost and + is; how to configure OWIN or where to set a breakpoint to see the request come in or the 503 response being sent out or how to get this logged or...

---

Edit: I have changed the code to read:

using Microsoft.Owin;
using Microsoft.Owin.Hosting;
using Owin;
using System;
using System.Threading.Tasks;

namespace TestApp
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            using (WebApp.Start<WebAppStartup>("http://localhost:8080/"))   // or: http://+:8080/
            {
                Console.WriteLine("Press [enter] to quit...");
                Console.ReadLine();
            }
        }
    }

    internal class WebAppStartup
    {
        public void Configuration(IAppBuilder app)
        {
            app.Use<TestMiddleware>();

            app.Run(context =>
            {
                context.Response.ContentType = "text/plain";
                Console.WriteLine("Sending response");
                return context.Response.WriteAsync("Hello, world.");
            });
        }
    }

    public class TestMiddleware : OwinMiddleware
    {
        public TestMiddleware(OwinMiddleware next) : base(next) { }

        public override async Task Invoke(IOwinContext context)
        {
            Console.WriteLine("Begin Request");
            await Next.Invoke(context);
            Console.WriteLine("End Request");
        }
    }
}

When I run the application with http://+:8080 I see the following appear in my console:

Begin Request
Sending response
End Request

And also "Hello, world" appears in my browser. I can set a breakpoint on the "begin request" line and it gets hit. Then, again, I change to http://localhost:8080 and ... HTTP Error 503. The service is unavailable.. Again. Console shows nothing (besides the obvious "Press [enter] to quit..."), the breakpoint is not hit.