Thanks for your help. But now I have another Problem: How I get the Value of the selcted Cell? Here is some code: 

 for (int i = 0; i < numberAppointments; i++)
            {
                UltraGridRow gridRow = new UltraGridRow();

                //Date in das Datum und die Uhrzeit zerlegen
                string[ dateAndTime = Convert.ToString(appointments[i].AppointmentTempDate).Split(' ');
                string date = dateAndTime[0];
                string time = dateAndTime[1];

                dateCell = new UltraGridCell();
                dateCell.Value = date;
                gridRow.Cells.Add(dateCell);

                timeCell = new UltraGridCell();
                timeCell.Value = time;
                gridRow.Cells.Add(timeCell);

                requestCell = new UltraGridCell();
                requestCell.Value = this.nodeSelected.ValuePath;
                gridRow.Cells.Add(requestCell);

                bookingCell = new UltraGridCell();
                bookingCell.Value = appointments[i];
                bookingCell.Text = "Termin buchen...";
                gridRow.Cells.Add(bookingCell);

                AppointmentSuggestionView.Rows.Add(gridRow);
            }

//This loop fills the Grid with data an the bookingCell.Value gets an appointment

 protected void AppointmentSuggestionView_ClickCellButton(object sender, CellEventArgs e)
        {
            UltraGridCell cell = e.Cell;

        }

//But the value of the CellEventArgsis null

How can I get the  value of the selected cell?

The event handler is should be added somewhere where the event handler will always be hooked up.  The handler is added when you hit your button, but hten on the postback, since the handler is not reattached, the event does not know how to get directed.

Add the handler in the form init, or add it at design time.  It's not like it would be hit if you don't have buttons in your grid.