ASP.NET HyperLink as updatepanel trigger
Control with ID 'X' being registered through RegisterAsyncPostBackControl or RegisterPostBackControl must implement either INamingContainer, IPostBackDataHandler, or IPostBackEventHandler.

<asp:ScriptManager ID="ScriptManager1" runat="server" />
<asp:UpdatePanel runat="server" id="UpdatePanel" updatemode="Conditional">
<Triggers>
    <asp:AsyncPostBackTrigger controlid="UpdateButton2" eventname="Click" />
</Triggers>
    <ContentTemplate>
        <asp:Repeater ID="rptDossiers" runat="server">
            <ItemTemplate>
                ...
            </ItemTemplate>
        </asp:Repeater>

        <asp:HyperLink NavigateUrl="#" runat="server" id="UpdateButton2" onclick="tousLesDossiers_Click">
            Tous les Dossiers
        </asp:HyperLink>
        <%--<asp:Button runat="server" id="UpdateButton2" onclick="tousLesDossiers_Click" text="Update" />--%>
    </ContentTemplate>
</asp:UpdatePanel>