404 error when send request to web api controller

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C#

ASP.NET

jQuery

API

MVC5

I am creating a ASP.NET MVC web site and as the title say i am trying to send a POST request to Web Api controller with jquery, but am always receiving a 404 error. The crazy thing is that in the .NET framework 4.5 the exact the same thing worked, but in the .NET framework 4.6.2 it really doesn't. I 've found a lot of threads in google explaining what it might go wrong, but none of these worked. So, here is my code now:

The Web Api Class:

[Authorize]
    public class CartController : ApiController
    {
        private ApplicationDbContext _context;

        public CartController()
        {
            _context = new ApplicationDbContext();
        }

        [HttpPost]
        public IHttpActionResult AddToCart(string productId)
        {
            if (!ModelState.IsValid || !ItemIsValid(productId))
                return BadRequest();

            var newCartItem = new ShoppingCartItem
            {
                ProductId = productId,
                UserId = User.Identity.GetUserId()
            };

            _context.ShoppingCartItems.Add(newCartItem);
            _context.SaveChanges();

            return Ok();
        }

        private bool ItemIsValid(string productId)
        {
            return Enumerable.Any(_context.Products, contextproduct => contextproduct.Id.ToString() == productId && contextproduct.NumberAvailable > 0);
        }
    }

My jQuery code:

$(".buy-button").click(function() {
                if (@User.Identity.IsAuthenticated.ToString().ToLower() === true) {
                    $.ajax({
                        url: "/api/cart",
                        method: "POST",
                        data: $(this).next().val()
                    }).done(function() {
                        alert("Product succesfully added to cart!");
                    });
                } else {
                    window.location.href = "@Url.Action("Login", "Account")";
                }
            });

The Global.asax code:

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

It would be great if someone could help me. I have spent over 6 hours on this bug, and i can't still find out what i am doing wrong.

What I have tried:

1) Some threads said that the order of the "calls" in the Global.asax matters, so i changed the position of '

RouteConfig.RegisterRoutes(RouteTable.Routes);

'
in every possible place.

2) Added Route Attributes in my Web Api action([Route("api/Cart/AddToCart")]) which i know from experience that is redundant ... and changed my jQuery code accordingly.

3) Instead of adding attributes, i added to Routes directly to the Web Api Config.

4) I also tried to call the Api with postman directly(i removed the Authorize attribute of course).

None of these worked.