Call the update of other update panel directly.
Try:
Private Sub objOpcGrps_DataChange(ByVal TransactionID As Integer, ByVal NumItems As Integer, ByRef ClientHandles As System.Array, ByRef ItemValues As System.Array, ByRef Qualities As System.Array, ByRef TimeStamps As System.Array) Handles objOpcGrp.DataChange
Dim i As Integer
For i = 1 To ClientHandles.Length
If ClientHandles(i) = 1 Then txtWaarde3.Text = ItemValues(i)
If ClientHandles(i) = 2 Then txtWaarde3.Text = ItemValues(i)
Next
UpdatePanel1.Update()
End Sub
The reason it does not work as you intend to is because of the page life-cycle and the way and when the click has been called for.