Ajax always returns HTTP error Error 404.15 - Not Found ASP.NET MVC 5

Question

I use ASP.NET MVC 5 and developed a website, when I execute the following Ajax request:

$.ajax({
    method: "POST",
    url: "/Galeria/AddGaleriaItem",
    data: { ID: galeriaID , ObjectString: JSON.stringify(obj) },
    dataType: "json"
})

It’s all set up right, but when it runs, Action is NEVER called...

[HttpPost]
public ActionResult AddGaleriaItem(string ID, string ObjectString)
{
    ImageIndexer image = JsonConvert.DeserializeObject<ImageIndexer>(ObjectString);
    TempData["idImageAdded"] = image.Code; 
    return Json(new { response = GaleriaCookieController.AddItem(image, ID).Successful });
}

AddGaleriaItem is inside the Controller Galeria, but I always get status: 404 and statusText: "Not Found": follows print of error:

I also receive a message via HTML like this :

HTTP Error 404.15 - Not Found

The request filtering module is configured to deny a request when the query character string is too long.

EDIT: I tested by decreasing the ObjectString, but still gives error 404... Only worked if I change POST to GET in both Ajax and Controller.