Unable to use MVC Controller with ASP.Net Web API 2

What I have so far (that works)

I have an ASP.Net Web API 2 project. Well, at least that's what I remember creating when I set up the project, I am not sure how to confirm that.

I am using Visual Studio 2017, and .Net Framework version 4.6.

In terms of the API side of things, this is all working great. The API controllers are fine, I can get data, post data, etc.

Just a bit of additional information in case it matters, I have added SignalR to the project which has been configured.

As it may be important, here are my various configuration files:

Global.asax.cs

protected void Application_Start()
{
    AreaRegistration.RegisterAllAreas();
    GlobalConfiguration.Configure(WebApiConfig.Register);
    FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
    RouteConfig.RegisterRoutes(RouteTable.Routes);
    BundleConfig.RegisterBundles(BundleTable.Bundles);
}

RouteConfig.cs

public static void RegisterRoutes(RouteCollection routes)
{
    routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

    routes.MapRoute(
        name: "Default",
        url: "{controller}/{action}/{id}",
        defaults: new { action = "Index", id = UrlParameter.Optional }
    );
}

WebApiConfig.cs

public static void Register(HttpConfiguration config)
{
    // Configure Web API to use only bearer token authentication.
    config.SuppressDefaultHostAuthentication();
    config.Filters.Add(new HostAuthenticationFilter(OAuthDefaults.AuthenticationType));

    // Web API routes
    config.MapHttpAttributeRoutes();

    config.Routes.MapHttpRoute(
        name: "DefaultApi",
        routeTemplate: "api/{controller}/{action}/{id}",
        defaults: new { id = RouteParameter.Optional }
    );
}

What I am trying to do (that doesn't work)

However, I want to add a HTML page so the user can view some information (just static HTML stuff, nothing special). So I have created a standard MVC controller with an action like so (and the Index.cshtml view is in the correct Views folder):

public class NotificationsController : Controller
{
    public ActionResult Index()
    {
        return View();
    }
}

The problem is that this action never gets run (I have a breakpoint).

What I have tried to identify the problem

Now I get at this point, it could be loads of different things, so here is what I have tried so far to debug the issue:

When I access the URL in a browser (e.g. http://localhost:59461/Notifications), I get:

localhost is currently unable to handle this request. HTTP ERROR 500

At first I thought maybe this is a routing issue, however in VS 2017 you can see that a request has failed for this action:

So surely the routing must be working correctly? Unfortunately, clicking the requests only confirms the 500 error and doesn't give any more information about the problem.

The only additional information I can find is in the Windows Event Viewer, in which I get the following error:

Application 'MACHINE/WEBROOT/APPHOST/PROJECTNAME' with physical root 'C:\PATH TO PROJECT FOLDER\' failed to start process with commandline '%LAUNCHER_PATH% %LAUNCHER_ARGS%', ErrorCode = '0x80070002 : 0.

But I have researched that error a lot and am yet to find a suitable solution or explanation for my problem.

I have also tried adding Application_Error but that isn't throwing any exceptions either.

At this point I don't know how to work out the cause of the problem. The only thing I can think of is that I need to configure something specifically to allow Web API projects to work with MVC controllers, but I can't find anything on that either.

What can I do to debug this problem correctly, and find the cause?