protected void btnaceptar_Click(object sender, ImageClickEventArgs e)
    {

        string MensajeError = "";
        int retornomensajes = 0;

        int retorno = 0;
        DTTarjetas d = null;
        if (hdopcionseleccionada.Value == "Add") // Here I check if the button acepts the Add button
        {
            // do something
        }
        else if (hdopcionseleccionada.Value == "Modify") // Here I check if the button acepts the Modify button
        {
            // do something
        }

        // When this method is called because of 'hdopcionseleccionada.Value == "Modify"' it updates the textbox
        // When this method is called because of 'hdopcionseleccionada.Value == "Add' it don't update the textbox
        CargarGrilla("Boton aceptar"); // Here occurs the problem
    }

    private void CargarGrilla(string metodo)
    {
        //Tiempos2 is the textbox
        Tiempos2.Text += "log 1: "+metodo +"nr";

        //..do something

        Tiempos2.Text += "log 2: " + metodo + "nr";

        //..do something

        Tiempos2.Text += "log 3: " + metodo+"nr";


    }