[asp.net]updatepanel refreshes only once

I have a very simple page:

<asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent">
    <%= DateTime.Now %>
    <asp:ScriptManager ID="ScriptManager1" runat="server">
    </asp:ScriptManager>
    <asp:UpdatePanel ID="UpdatePanel1" runat="server">
        <ContentTemplate>
            <%= DateTime.Now %>
        </ContentTemplate>
    </asp:UpdatePanel>
</asp:Content>

This version work as I expected. From firebug I can refresh the updatepanel everytime using __doPostBack('MainContent_UpdatePanel1',''). The second version has only one new control, this is AsyncFileUpload from ajax control toolkit. This time using javascript I can only refresh the updatepanel once. Why is this happening?

<asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent">
    <%= DateTime.Now %>
    <asp:A开发者_StackOverflow社区syncFileUpload ID="AsyncFileUpload1" runat="server" />
    <asp:ScriptManager ID="ScriptManager1" runat="server">
    </asp:ScriptManager>
    <asp:UpdatePanel ID="UpdatePanel1" runat="server">
        <ContentTemplate>
            <%= DateTime.Now %>
        </ContentTemplate>
    </asp:UpdatePanel>
</asp:Content>

---

There seem to be a couple of issue re triggering postbacks with the upload control on the page

http://ajaxcontroltoolkit.codeplex.com/workitem/25475