Autheracation error: Key cannot be null. Parameter name: key - asp.net-mvc-4

Question

I keep getting the error Key cannot be null. Parameter name: key. It looks like it looses the authCookie, but the Request.IsAuthenticated still is set to true. It's driving my crazy. Also i'm allready handling if the user is not logged in. But the error still appears.
An place where it fails is fore example here
#if (User.IsInRole("Admin"))
{
<button class="button-green white">Save</button>
}
But if look at what the user has of properties it still says IsAuthenticated = true and the name and Role is also correct, and the ticket is still valid.
This is the callstack
System.ArgumentNullException was unhandled by user code
HResult=-2147467261
Message=Key cannot be null.
Parameter name: key
Source=System
ParamName=key
StackTrace:
at System.Collections.Specialized.HybridDictionary.get_Item(Object key)
at System.Web.Security.RolePrincipal.IsInRole(String role)
at ASP._Page_Views_Admin_EditText_cshtml.Execute() in d:\SubVersion\Oline\Project1\Source\Oline.Portal\Views\Admin\EditText.cshtml:line 78
at System.Web.WebPages.WebPageBase.ExecutePageHierarchy()
at System.Web.Mvc.WebViewPage.ExecutePageHierarchy()
at System.Web.WebPages.StartPage.RunPage()
at System.Web.WebPages.StartPage.ExecutePageHierarchy()
at System.Web.WebPages.WebPageBase.ExecutePageHierarchy(WebPageContext pageContext, TextWriter writer, WebPageRenderingBase startPage)
at System.Web.Mvc.RazorView.RenderView(ViewContext viewContext, TextWriter writer, Object instance)
at System.Web.Mvc.BuildManagerCompiledView.Render(ViewContext viewContext, TextWriter writer)
at System.Web.Mvc.ViewResultBase.ExecuteResult(ControllerContext context)
at System.Web.Mvc.ControllerActionInvoker.InvokeActionResult(ControllerContext controllerContext, ActionResult actionResult)
at System.Web.Mvc.ControllerActionInvoker.<>c__DisplayClass1a.<InvokeActionResultWithFilters>b__17()
at System.Web.Mvc.ControllerActionInvoker.InvokeActionResultFilter(IResultFilter filter, ResultExecutingContext preContext, Func`1 continuation)
InnerException:
This is my Custom RoleProvier
public override string[] GetRolesForUser(string username)
{
var user = (string)HttpContext.Current.Session["UserRoles"];
var ret = new[] { user };
return ret;
}

Answer 1

The solution to the problem was, this section var user = (string)HttpContext.Current.Session["UserRoles"];, that becomes null. Special then I go back and fort, changing things in Visual Studio 2012. Thanks for the suggestions :)