[AcceptVerbs(HttpVerbs.Post)]
    public ActionResult LaadVerrichtingenIn() {
        int[] intArray = Array.ConvertAll<String, int>(Request.Form["selectedObjects"].Split(','), new Converter<String, int>(Convert.ToInt32));
        List<Verrichting> gekozenVerrichtingen = new List<Verrichting>();

        foreach(int i in intArray){
            base._entities.AddToVerrichtingSet(((Dictionary<int, Verrichting>)Session["ingelezenVerrichtingen"])[i]);
            gekozenVerrichtingen.Add(((Dictionary<int, Verrichting>)Session["ingelezenVerrichtingen"])[i]);
        }
        Session["ingelezenVerrichtingen"] = null;
        base._entities.SaveChanges(); //Exception occurs here

        return View("IngeladenVerrichtingen");
    }

Thread.CurrentThread.CurrentCulture = new CultureInfo("en-US");