ขอความช่วยเหลือครับ ปัญหา An error occurred while processing your request

public ActionResult Index(int? page, string criSearch, string criLoc, string txtSearch)
        {            
            userLogin = (tbuser) HttpContext.Session["userLogin"];

            if (userLogin != null)
            {
                ViewBag.UserName = userLogin.username;
                ViewBag.userLev = userLogin.level;
                ViewBag.Login = true;
            }
            else
            {
                ViewBag.UserName = "Guest";
                ViewBag.userLev = 9;
                ViewBag.Login = false;
            }

            ViewBag.criSearch = criSearch;
            ViewBag.criLoc = criLoc;
            ViewBag.txtSearch = txtSearch;

            var model = this._db.tbexts.ToList();
            int pageSize = 20;
            int pageNumber = (page ?? 1);
            dropdownList();            

            if (criLoc != "" && criLoc != null)
                model = model.Where(c => c.extLoc == Convert.ToInt32(criLoc)).ToList();

            if (criSearch != "" && criSearch != null)
            {
                if (criSearch == "1")  // Extension
                    model = model.Where(c => c.extType == 1 ).ToList();
                else if (criSearch == "3")       // Department
                    model = model.Where(c => c.extDept != 0 ).ToList();
                    //model = model.Where(c => c.extBranch == 2).ToList();
                else if (criSearch == "2")  // Extension
                    model = model.Where(c => c.extType == 1 ).ToList();
                else if (criSearch == "4")  // Tel
                    model = model.Where(c => c.extType == 3 ).ToList();
                else if (criSearch == "5")  // Fax
                    model = model.Where(c => c.extType == 2 ).ToList();
            }

            if (txtSearch != "" && txtSearch != null)
            {
                if (criSearch == "1") // Search in name (EN,TH,Nick)
                    model = model.Where(c => c.extFEN.ToLower().Contains(txtSearch.ToLower()) || c.extLEN.ToLower().Contains(txtSearch.ToLower()) || c.extFTH.ToLower().Contains(txtSearch.ToLower()) || c.extLTH.ToLower().Contains(txtSearch.ToLower()) || c.extNick.ToLower().Contains(txtSearch.ToLower())).ToList();
                else if (criSearch == "3") // Department
                    model = model.Where(c => c.tbbranch.branchEN.ToLower().Contains(txtSearch.ToLower())).ToList();
                else if (criSearch == "2") // Extension
                    model = model.Where(c => c.extNO.ToLower().Contains(txtSearch.ToLower())).ToList();
                else if (criSearch == "4") // Tel
                    model = model.Where(c => c.extNO.ToLower().Contains(txtSearch.ToLower())).ToList();
                else if (criSearch == "5") // Fax
                    model = model.Where(c => c.extNO.ToLower().Contains(txtSearch.ToLower())).ToList();
                else // Search in name (EN,TH,Nick)
                    model = model.Where(c => c.extFEN.ToLower().Contains(txtSearch.ToLower()) || 
                                             c.extLEN.ToLower().Contains(txtSearch.ToLower()) ||
                                             c.extFTH.ToLower().Contains(txtSearch.ToLower()) || 
                                             c.extLTH.ToLower().Contains(txtSearch.ToLower()) ||
                                             c.extNick.ToLower().Contains(txtSearch.ToLower()) ||
                                             c.extNO.ToLower().Contains(txtSearch.ToLower()) ||
                                             c.tbbranch.branchEN.ToLower().Contains(txtSearch.ToLower()) ||
                                             c.extNO.ToLower().Contains(txtSearch.ToLower())
                                             ).ToList();
            }

            return View(model.ToPagedList(pageNumber, pageSize));
        }