[a33ik]

Измените

                    try
                    {
                        AssignResponse assignResponse = (AssignResponse)KSservice.Execute(assign);
                    }
                    catch (Exception ex)
                    {
                        //TODO: Exceptionhandling
                    }

на

                    try
                    {
                        AssignResponse assignResponse = (AssignResponse)KSservice.Execute(assign);
                    }
                    catch (SoapException ex)
                    {
throw new Exception(ex.Detail.InnerText);
                    }

Это даст детали по ошибке.