Created Unassigned: Ajax modal popup extender does not popup [27465]

I'm using asp.net framework with ajaxcontroltoolkit 7.0607. The modal popup does not 'popup'The previous release worked find:

Here's the markup

<asp:Panel ID="PanelDiscardChanges" runat="server">
<table>
<tr>
<td colspan="2" align="center">
<asp:Label ID="label32" runat="server"
Text="Discard Changes?" Font-Bold="True"
Width="190px" ForeColor="Red" BorderStyle="Outset" Height="25px"></asp:Label>
</td>

</tr>
<tr>

<td align="center">
<asp:Button ID="btnDiscard_Cancel" runat="server" Text="No"
BorderStyle="Solid" Height="30px" Width="93px" CausesValidation="False"
ValidationGroup="UpdateWeight" />
</td>

<td align="center">
<asp:Button ID="btnDiscard_Continue" runat="server" Text="Yes"
BorderStyle="Solid" Height="30px" Width="93px" CausesValidation="False"
ValidationGroup="UpdateWeight" />
</td>

</tr>
</table>

</asp:Panel>
<asp:ModalPopupExtender ID="PanelDiscardChanges_ModalPopupExtender2"
runat="server" DynamicServicePath="" Enabled="True"
TargetControlID="btnDiscard_Modal" PopupControlID="PanelDiscardChanges" >
</asp:ModalPopupExtender>



<div style="visibility:hidden">
<asp:Button ID="btnDiscard_Modal" runat="server" Text="Discard Changes"
BorderStyle="Dotted" />
</div>

I use the show method for the extender as shown

If Request.Form("User$MainContent$btnCancel") <> "" Then
labelShipmentClosed.Text = "Cancel Update?."

PanelDiscardChanges_ModalPopupExtender2.Show()

MultiView1.ActiveViewIndex = 2

Exit Sub
End If