Hi danparks,
It works fine for me, please test the example below:
If you still get this issue, please create a page can reproduce this issue and send it to me.
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<%@ Page Language="C#" %>
<%@ Register Namespace="AjaxControlToolkit" Assembly="AjaxControlToolkit" TagPrefix="ajax" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "
http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="
http://www.w3.org/1999/xhtml">
<head runat="server">
<title>test</title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<ajax:CollapsiblePanelExtender ID="CollapsiblePanelExtender1" runat="server" TargetControlID="Panel1"
CollapseControlID="Button1" ExpandControlID="Button1">
</ajax:CollapsiblePanelExtender>
<asp:Button ID="Button1" runat="server" Text="HideAndShow" />
<asp:Panel ID="Panel1" runat="server" Height="400" Width="400">
<CuteWebUI:Uploader ID="uu1" runat="server" InsertButtonID="Button2">
</CuteWebUI:Uploader>
<asp:Button ID="Button2" runat="server" Text="upload" />
</asp:Panel>
</div>
</form>
</body>
</html>
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Regards,
Ken