Web API - HTTP error 500.19 - internal server error

public class EmployeeController : ApiController
    {
        EmployeeBusiness employeeBusiness = new EmployeeBusiness();
        public List<EmployeeModel> GetAllEmployee()
        {
            var employeeData = employeeBusiness.GetAllEmployee();
            return employeeData;
        }
    }

public class EmployeeBusiness
    {
        CampaignAPIEntities dbcontext = new CampaignAPIEntities();

        private List<EmployeeModel> employeeData = new List<EmployeeModel>();
        EmployeeModel employeeModel = new EmployeeModel();
        public List<EmployeeModel> GetAllEmployee()
        {
            //employeeData = new List<EmployeeModel>();
            var _employee = dbcontext.Employees.Select(e => e);
            foreach (var item in _employee)
            {
                employeeModel.ID = item.ID;
                employeeModel.Name = item.Name;
                employeeModel.PhoneNumber = item.PhoneNo;
                employeeModel.DeviceID = item.DeviceID;
                employeeModel.Status = item.Status;

                //-- Adding model to list --
                employeeData.Add(employeeModel);
            }
            return employeeData;
        }
    }