The HttpUnhandledException is 'No UpdatePanel found for AssociatedUpdatePanelID 'UpdatePanel1'.
<ajaxToolkit:Accordion ID="Accordion1" runat="server">
<Panes>
<ajaxToolkit:AccordionPane ID="AccordianPane1" runat="server">
<Header>
<asp:Label ID="Label1" runat="server" Text="Header" />
<asp:UpdateProgress ID="UpdateProgress1" runat="server" AssociatedUpdatePanelID="UpdatePanel1">
<ProgressTemplate>
<asp:Label runat="server" Text="Updating..." />
</ProgressTemplate>
</asp:UpdateProgress>
</Header>
<Content>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp:Button ID="Button1" runat="server" Text="Submit" />
</ContentTemplate>
</eporus:ContainerUpdatePanel>
</Content>
</ajaxToolkit:AccordionPane>
</Panes>
</
ajaxToolkit:Accordion>
Nathan