user controls within Container - Error on Postback

Hi,
I have a webpage (webform1.aspx) which has a splitter which gives us left panel and right panel. In the left panel, I have a treeview which depends on an xmldatasource.  In the right panel, I have two containers which one containing 2 or more custom user controls.
WebForm1.aspx Code

<%@ Page Language="vb" CodeBehind="WebForm1.aspx.vb" AutoEventWireup="false" Inherits="Monitor.WebForm1"%>
<%@ Register TagPrefix="obspl" Namespace="OboutInc.Splitter2" Assembly="obout_Splitter2_Net" %>
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>

<script runat="server">
Sub SetNodes(ByVal node As TreeNode)
...