Adding new controller gives HTTP error - asp.net-mvc

Question

In my ASP.NET MVC 5 application, I used to have a controller called Alpha, which I played around with for a while, and then ended up deleting it. Now, I've decided that I want to have another controller called Alpha, with different functionality from before.
So, I have added a new controller, named it Alpha, and added an Index view for this controller. But when I go to localhost:IP/Alpha, I get the following error:
HTTP Error 403.14 - Forbidden
The Web server is configured to not list the contents of this directory.
All I have done though, is add the controller with the code, and then add an Index view with the default code.
The strange thing, is that if I add a controller and a view in the same way, but call it anything other than Alpha, then the application works as would be expected, and Alpha's Index view is displayed.
So I am assuming there is something remaining in the system from the old Alpha controller (which may have originally been buggy, I cannot remember), which is conflicting with this new Alpha. Any ideas on how I might investigate this further?
As requested by Jason Evans, here is my Global.asax.cs file:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Http;
using System.Web.Mvc;
using System.Web.Optimization;
using System.Web.Routing;
namespace Poseidon
{
// Note: For instructions on enabling IIS6 or IIS7 classic mode,
// visit http://go.microsoft.com/?LinkId=9394801
public class MvcApplication : System.Web.HttpApplication
{
protected void Application_Start()
{
AreaRegistration.RegisterAllAreas();
WebApiConfig.Register(GlobalConfiguration.Configuration);
FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
RouteConfig.RegisterRoutes(RouteTable.Routes);
BundleConfig.RegisterBundles(BundleTable.Bundles);
AuthConfig.RegisterAuth();
}
}
}
As requested by Jason Evans, here is my RoutConfig::RegisterRoutes function:
public static void RegisterRoutes(RouteCollection routes)
{
routes.IgnoreRoute("{resource}.axd/{*pathInfo}");
routes.MapRoute(
name: "Default",
url: "{controller}/{action}/{id}",
defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional }
);
}