title: ModalPopup is not working
body: <p>Since i changed my website from .NET 4.0 to 3.5 ModalPopup is not working.
  The problem is there's no error message showing. So i can't figure out how to fix it.
  Has anyone had the same problem before?</p>
  
  <pre><code>&lt;%@ Page Language="C#" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" %&gt;
  
  &lt;%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %&gt;
  
  &lt;!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"&gt;
  
  &lt;html xmlns="http://www.w3.org/1999/xhtml"&gt;
  &lt;head runat="server"&gt;
      &lt;title&gt;&lt;/title&gt;
  &lt;/head&gt;
  &lt;body&gt;
      &lt;form id="form1" runat="server"&gt;
      &lt;asp:ScriptManager ID="ScriptManager1" runat="server"&gt;
      &lt;/asp:ScriptManager&gt;
      &lt;div&gt;
          &lt;asp:Button ID="btnpopup" runat="server" Text="Button" /&gt; 
  
          &lt;asp:ModalPopupExtender ID="mpe" runat="server" TargetControlID="btnpopup" PopupControlID="pnlpopup" 
              CancelControlID="btnCancelpopup" EnableViewState="true" DropShadow="true" /&gt;
  
              &lt;asp:Panel ID="pnlpopup" runat="server" Width="400px"&gt;
                  test
              &lt;asp:Button ID="btnCancelpopup" runat="server" Text="Button" /&gt;
          &lt;/asp:Panel&gt;
      &lt;/div&gt;
      &lt;/form&gt;
  &lt;/body&gt;
  &lt;/html&gt;
  </code></pre>