You need to have a URL Epi can route to a page data object that you've added to the page tree.

If LoginPage is a Epi page type you need to use a URL of such a page.

Something like http://localhost:53067/my-cool-login-page/

I'm having the same problem. I'm a complete epi newbie except for an intensive boot camp training. 

I've created a login page, viewmodel, controller and view according to this article

https://support.episerver.com/hc/en-us/articles/115002559451

Can you explain what you mean in the first sentence of your answer? How do I add my page to the page tree?  And how do I know what my page is called? Isn't is just login? 

Here's the definition of my login page:

[ContentType(DisplayName = "LoginPage", GUID = "a1c77b97-95a6-447d-af9a-4b79b8770040", Description = "Login Page")]
[AvailableContentTypes(Availability = Availability.Specific,
IncludeOn = new[] { typeof(StartPage) },
ExcludeOn = new[] { typeof(ContainerPage), typeof(ProductPage), typeof(StandardPage), typeof(ISearchPage), typeof(LandingPage) })]
public class LoginPage : StandardPage
{

}

Thanks for your help.

1. Go to Edit mode.
2. Add a new page of type "Login Page" below your start page.
3. Check which URL it got.
4. Set that URL, something like /en/my-login-page/ as the default login page in web.config.

This setup has some drawbacks since you need to set public access for that page you added, it not you will cause a redirect loop. Use /util/login.aspx as a backdoor if this has happend.

Thanks for your help. That did the trick!

The above solution assumes that your login page has a specific name-in-url set. To avoid that dependency you can do something like this.

In Global.asax.cs:

protected override void RegisterRoutes(RouteCollection routes)
{
    // login page route
    routes.MapRoute("Login", "login", new { Controller = "SignIn", action = "Index" });
    base.RegisterRoutes(routes);
}

In SignInPageController:

public override ActionResult Index(SignInPage currentPage)
{
    // if you come here from custom route and are missing currentPage
    if (currentPage == null)
        currentPage = PageHelper.SiteSettingsPage.SignInPage.Get<SignInPage>();

    var model _signInPageFactory.Service.Create(currentPage);
    return View("Index", model);
}

However, you will need to have some kind of global site config where you reference your Login page so you can fetch it programatically.