UpdatePanel does not work when AjaxControlToolkit lib is included - asp.net

Question

I work on a simple ASP Webform project (asp framework 4.0), and use VS 2010.
I have a some pages on my website where I want to use UpdatePanels.
On another page of my website, i need to use ModalPopupExtender control wich is included in the AjaxControlToolkit lib.
The problem that i encountered is that my UpdatePanels do not work at all if I include the AjaxControlToolkit library in my project (no need to implement any component of this library, just adding a reference to it in my project).
If I remove the reference to the lib, it works fine.
Do you have an idea on how to concile those both elements ? UpdatePanel and AjaxControlToolkit lib ?
Thanks a lot for your advice
Here is the simple UpdatePanel code I use:
<asp:ScriptManager ID="ScriptManager1" runat="server" />
<asp:Repeater ID="Repeater1" runat="server" OnItemCommand="Repeater1_ItemCommand" OnItemDataBound="Repeater1_ItemDataBound">
<ItemTemplate>
<asp:LinkButton ID="lnk" runat="server" Text="show" CommandName="show"></asp:LinkButton>
</ItemTemplate>
</asp:Repeater>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
<asp:Label runat="server" ID="label1"></asp:Label>
</ContentTemplate>
<Triggers>
<asp:AsyncPostBackTrigger ControlID="Repeater1" EventName="ItemCommand" />
</Triggers>
</asp:UpdatePanel>
In code behind, I use this :
protected void Page_Load(object sender, EventArgs e)
{
Repeater1.DataSource = new List<String>(){"zero", "un", "deux", "trois","quatre"};
Repeater1.DataBind();
}
protected void Repeater1_ItemDataBound(object sender, RepeaterItemEventArgs e)
{
LinkButton lnk = ((LinkButton)e.Item.FindControl("lnk"));
lnk.CommandArgument = e.Item.ItemIndex.ToString();
}
protected void Repeater1_ItemCommand(object source, RepeaterCommandEventArgs e)
{
if (e.CommandName == "show")
label1.Text = "You click" + e.CommandArgument;
}

Answer 1

I found this post because i was the same problem when added a reference for AjaxToolKit.dll,
then was looking what could be the problem then found this with the comment to see the javascript errors and was the solution :D
I had in my code a script to get the start of the refresh
var prm = Sys.WebForms.PageRequestManager.getInstance();
prm.add_beginRequest(BeginRequestHandler);
prm.add_endRequest(EndRequestHandler);
function BeginRequestHandler(sender, args)
{
var elementID = sender._activeElement.id;
...
}
was getting "TypeError: sender._activeElement is undefined"
then changed the
var elementID = sender._activeElement.id;
to
var elementID = sender._postBackSettings.sourceElement.id;
and solved my problem.
Without the reference to the dll was working fine and when I remove it.
With the reference I had to change the properties of the element.