AJAX toolkit ModalPopupExtender does not display under all conditions

We have just rolled out a web application that uses the AJAX ModalPopupExtender. The site has gone through our testing and runs fiine on Firefox 2/3 and IE 6/7. However, one Windows XP/IE6 client does not see the popup(s) at all and a screen shot shows the "errors on page icon" (we have not been able to get this information).
Other clients are experiencing the same problem so it may not be limited to this platform or browser.
We have a large error message in noscript tags to catch Scripting disabled and this is not being seen by the client.
We use a Windows XP/IE6 PC for our testing so we are thinking this is a browser setting/firewall issue.
We went though the IE security settings and disabled all enabled settings one at a time, but could not reproduce the problem.
One popup is triggered in client code and another in server side code, does not seem to make any difference to the issue.
We cannot reproduce in our development environment.
we are using version 1.0 of the toolkit with ASP.Net 2.0.
The AJAXControlToolkit.dll version is 1.0.20229.0.
The AJAXExtensionsToolbox.dll version is 1.0.61025.0.
I would appreciate any suggestions that would fix or help diagnose the issue.Start Free Trial

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<ajaxToolkit:ModalPopupExtender ID="ModalPopupExtender" runat="server" 
            BehaviorID="showCreateSupervisor"
            TargetControlID="hiddenTargetControlForModalPopup"
            PopupControlID="PanelModalSupervisor" 
            BackgroundCssClass="modalBackground"
            CancelControlID="ButtonCancel"
            PopupDragHandleControlID="PanelConfirmTitle" />
 
<!-- Using a hidden button  as the target control id -->
 
<asp:Button runat="server" ID="hiddenTargetControlForModalPopup" style="display:none"/>