同时调用两个ajax会导致两个内部服务器错误

我有两个函数执行两个ajax调用。如果我调用一个函数而不调用另一个代码正常执行。但是如果我同时调用它们，我会得到两个内部服务器错误消息。我想每个功能一个。

这是我的代码:

   $(document).ready(function(){
         CategoryChangeState(@Model.CatId , subcategoryId);
         SubategoryChangeState(@Model.SubcatId);
    })
    public ActionResult ReturnListOfSubcategories( FormCollection collection ) {
            string categoryId = collection["result"];
            var subcategories = ProductManagerHelperClass.ReturnSubcategories(categoryId);
            return Json(subcategories);
    }
    public ActionResult ReturnListOfBrands() {
          var brands = ProductManagerHelperClass.ReturnBrands();
          return Json(brands);
    }
function CategoryChangeState(value ,  editPage) {
        .....
        $.ajax({
            type: "POST",
            url: "/ProductManager/ReturnListOfSubcategories",
            data: { result: value },
            datatype: "json",
            error: function (jqXHR, textStatus, errorThrown) {
                alert(errorThrown);
            }
        });
        ...
}
function SubategoryChangeState(value) {
         ....
         $.ajax({
            type: "POST",
            url: "/ProductManager/ReturnListOfBrands",
            datatype: "json",
            error: function (jqXHR, textStatus, errorThrown) {
                alert(errorThrown);
            }
        });
       ....
}

为什么我得到这些错误，我怎么解决它?

编辑

在调试时，我发现在这部分代码中:

 public static Dictionary<string , string> ReturnSubcategories(string categoryId)
    {
        int catId = int.Parse(categoryId);
        var subcategories = (from s in dataContext.SubCategories
                             where s.CatId == catId
                             select new
                                        {
                                            s.SubCatId,
                                            s.SubCatName
                                        }).ToDictionary(x => x.SubCatId.ToString(), x => x.SubCatName);
        return subcategories;
    } 

linq查询抛出异常:

InvalidOperationException ExecuteReader requires an open and available Connection. The connection's current state is closed.

同样，此异常仅在同时调用