Postback error on ASP page

Greetings all

I am experiencing a Invalid Postback error on my page (code included under snippets).

The steps are:

1.  Browse the default page

2.  Tab to a selected month and select a day (note: if you select a day on the curretn calendar
     page, then error doesn't occur)

3.  Click the hyperlink to go to a diferent page.

4.  Click the browser's "<--" to return to the page
     At this stage, the view is that which we saw when we first browsed the page

5.  Click the "<" button on the calendar to go to the month from which I previously selected a
     date  --- POST BACK ERROR


 A few notes ...

a.  I'm using IExplorer

b.  If I press <F5> after Step2 and then proceed a normal, then it works fine

c. I am using the following code to block caching for Fox. If I apply this to IE, then it
   also works.
                           if (Request.Browser.MSDomVersion.Major == 0) // Non IE Browser?)
                              Response.Cache.SetNoStore(); // No client side cashing for non IE browsers


What am I missing? What do I need to change, to get it working without?

In advance, thanks!

    allanmark

DEFAULT PAGE SOURCE:
 
<%@ Page Language="C#" MasterPageFile="~/MimeMaster01.master" AutoEventWireup="true" CodeFile="Default.aspx.cs" Inherits="_Default" Title="Untitled Page" %>
 
<asp:Content ID="Content1" ContentPlaceHolderID="leftnav" Runat="Server">
 
    <ajaxext:ScriptManagerProxy ID="ScriptManagerProxy1" runat="server">
    </ajaxext:ScriptManagerProxy>
 
    <center>
    <asp:HyperLink  ID="hypContacts" runat="server" CssClass="hypLink" NavigateUrl="~/Miscellaneous/Contacts.aspx" >Contacts</asp:HyperLink>
    </center>   
    <br />    
    <br />
 
    <ajaxext:UpdatePanel ID="UpdatePanel1" runat="server">
    <ContentTemplate>
         
        <center>
        <asp:Calendar ID="calEvents" runat="server" BackColor="White" BorderColor="#999999"
            CellPadding="4" DayNameFormat="Shortest" Font-Names="Verdana" Font-Size="8pt"
            ForeColor="Black" Height="180px" Width="200px" OnSelectionChanged="calEvents_SelectionChanged">
            <SelectedDayStyle BackColor="#ffc0cb" Font-Bold="True" ForeColor="White"  />
            <TodayDayStyle BackColor="#ffc0cb" ForeColor="Black" />
            <SelectorStyle BackColor="#CCCCCC" />
            <WeekendDayStyle BackColor="#CD8C95" />
            <OtherMonthDayStyle ForeColor="#808080" />
            <NextPrevStyle VerticalAlign="Bottom" />
            <DayHeaderStyle BackColor="#ffb6c1" Font-Bold="True" Font-Size="7pt" />
            <TitleStyle BackColor="#ffc0ca" BorderColor="Black" Font-Bold="True" />
        </asp:Calendar>
        </center>
        
        <br />
        <center>
        <h5 class="h5header" >Events for the day:</h5>
        <asp:ListBox ID="lbxEvents" runat="server" CssClass="listbox"  ></asp:ListBox>
        </center>
        
    </ContentTemplate>
    </ajaxext:UpdatePanel>
    
 
</asp:Content>
 
<asp:Content ID="Content2" ContentPlaceHolderID="righnav" Runat="Server">
 
<h3>Welcome!</h3>
<p>This is the administration site of Marriage Encounter (ME) - Western Cape. Access is restricted to ME members who are a part of the administration team. If you are a team member but do not have access, please contact the Administrator (click the "Contacts" link in the left pane to be taken to our contacts page) and request the necessary clearance.</p>
<br />
<p><i>The Admin Team!</i></p>
    
</asp:Content>
 
 
 
DEFAULT PAGE CODE BEHIND:
 
using System;
using System.Data;
using System.Text;
using System.Configuration;
using System.Collections;
using System.Collections.Generic;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using BusinessLogic;
using DataTransferObjects;
 
public partial class _Default : System.Web.UI.Page
{
    private List<ME_Event> dailyEvents;
 
    private bool loggedIn;
 
    protected void Page_Load(object sender, EventArgs e)
    {
        bool needToCallEvents;
 
 
        needToCallEvents = false;
 
        if (Request.Browser.MSDomVersion.Major == 0) // Non IE Browser?)
          Response.Cache.SetNoStore(); // No client side cashing for non IE browsers
 
        // Load the Page-specific stylesheet.
        HtmlLink lnk = (HtmlLink)Master.FindControl("cssLink");
        if (lnk != null)
            lnk.Href = "~/CSS/Default.css";
 
        if (Page.IsPostBack)
        {
            if (Session["loggedIn_Session"] != null)
            {
                if ((bool)Session["loggedIn_Session"] != User.Identity.IsAuthenticated)
                {
                    loggedIn = User.Identity.IsAuthenticated;
                    Session["loggedIn_Session"] = loggedIn;
                    FetchDailyEvents();
                }
                else
                {
                    loggedIn = (bool)Session["loggedIn_Session"];
 
                    dailyEvents = (List<ME_Event>)Session["dailyEvents_Session"];
                    lbxEvents.DataSource = dailyEvents;
                    lbxEvents.DataBind();
 
                    calEvents.VisibleDate = calEvents.SelectedDate;
                }
 
            }
            else
            {
                loggedIn = User.Identity.IsAuthenticated;
                Session["loggedIn_Session"] = loggedIn;
                FetchDailyEvents();
            }
        }
        else
        {            
            // Not PostBack
            needToCallEvents = false;
 
            if (Session["loggedIn_Session"] != null)
            {
                if ((bool)Session["loggedIn_Session"] != User.Identity.IsAuthenticated)
                {
                    loggedIn = User.Identity.IsAuthenticated;
                    Session["loggedIn_Session"] = loggedIn;
                    needToCallEvents = true;
                }
                else
                {
                    loggedIn = (bool)Session["loggedIn_Session"];
                }
 
                if (Session["selectedDate_Session"] != null)
                {
                    calEvents.SelectedDate = (DateTime)Session["selectedDate_Session"];
                    calEvents.VisibleDate = calEvents.SelectedDate;
                }
                else
                {
                    calEvents.SelectedDate = calEvents.TodaysDate;
                    calEvents.VisibleDate = calEvents.SelectedDate;
                }
            }
            else
            {
                loggedIn = User.Identity.IsAuthenticated;
                calEvents.SelectedDate = calEvents.TodaysDate;
                needToCallEvents = true;
            }
 
                FetchDailyEvents();
 
 
        }
 
        
    }
 
    protected void calEvents_SelectionChanged(object sender, EventArgs e)
    {
        FetchDailyEvents();
 
    }
 
    private void FetchDailyEvents()
    {     
        lbxEvents.DataSource = "";
        dailyEvents = null;
 
        if (loggedIn)
        {
            dailyEvents = BusinessLogic.Business.GetEvents(calEvents.SelectedDate, "Y");
        }
        else
        {
            dailyEvents = BusinessLogic.Business.GetEvents(calEvents.SelectedDate, "N");
        }
 
        lbxEvents.DataSource = dailyEvents;
        lbxEvents.DataTextField = "Title";
        lbxEvents.DataValueField = "Id";
        lbxEvents.DataBind();
 
        Session["selectedDate_Session"] = calEvents.SelectedDate;
        calEvents.VisibleDate = calEvents.SelectedDate;
 
        Session["dailyEvents_Session"] = dailyEvents;
    }
 
}
 
 
MASTER PAGE SOURCE:
 
<%@ Master Language="C#" AutoEventWireup="true" CodeFile="MimeMaster01.master.cs" Inherits="MimeMaster01" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
 
<html xmlns="http://www.w3.org/1999/xhtml" >
 
<head id="Head1" runat="server">
    
    <title>Mime Master</title>
    
     <link id="cssLink" rel="stylesheet" type="text/css" runat="server"
    href="CSS/Mime.CSS" />  
    
</head>
 
<body id="MasterBody" runat="server" >
 
<form id="form1" runat="server" style="height: 100%;">
    
<div id="container" >
 
    <a name="theTop" ></a>  
    
	<div id="header">
	<!-- Header start -->		  
          		  
		  <ajaxext:ScriptManager ID="scriptmanager1" runat="server">
          </ajaxext:ScriptManager>          
          
          <asp:LoginName  ID="lgUser" runat="server" CssClass="loginName" FormatString="Member: {0}"/>
          
          <center>
		  <h1>Marriage Encounter - Western Cape</h1>
		  <h5>(Administration Management)</h5>
          </center>
          
          <asp:Label ID="lblFollowup" runat="server" CssClass="followup" Visible="false">** Follow-ups Outstanding **</asp:Label>
                         
          <asp:Image ID="headerPict" runat="server" CssClass="headerPic" ImageUrl="~/Graphics/ME05.Gif" />
 
          <center><ul id='menu' style="width: 67%">
                <li ><a href='<%=ResolveUrl("Default.aspx")%>'  id='menuHome' >Home</a></li>
                <li ><a href='<%=ResolveUrl("Default2.aspx")%>' id='menuPeople' >People</a></li>
                <li ><a href='<%=ResolveUrl("Default2.aspx")%>' id='menuCircles' >Circles</a></li>
                <li ><a href='<%=ResolveUrl("Default2.aspx")%>' id='menuWeekends' >Weekends</a></li>
                <li ><a href='<%=ResolveUrl("Default2.aspx")%>' id='menuEvents' >Events</a></li>
                <li ><a href='<%=ResolveUrl("Default2.aspx")%>' id='menuGeneral' >General</a></li>
                <li ><a href='<%=ResolveUrl("Default2.aspx")%>' id='menuPortfolios' >Portfolio</a></li>
                <li ><a href='<%=ResolveUrl("Default2.aspx")%>' id='menuUsers' >Users</a></li>
              </ul>
              
              <asp:LoginStatus ID="logOn" runat="server" CssClass="loginStat" />              
            <!-- Give a little space -->
            <div style="height:15px"></div>
 
              <hr  width="96%" class="redline"/>
          </center>
 
          
	<!-- Header end -->
	</div>
	
	<div id="body">
    <!-- Body start -->
    
	    <div id="dummy" style="position:relative; height: 40px"></div>
		
        <div id="wrapperDiv" class="wrapper"> 	
	  
	    <div id="left" class="leftnav">	
		       <asp:ContentPlaceHolder ID="leftnav" runat="server">
                </asp:ContentPlaceHolder>
        </div>
        
        <div id="right" class="rightnav">	
              <asp:ContentPlaceHolder ID="righnav" runat="server">
              </asp:ContentPlaceHolder>
        </div>
      
      <!-- <div id="wrapper"> 	 -->
      </div>
        
        <!-- Give a little space -->
        <div style="height:45px"></div>
        
    <!-- Body end -->
	</div>
	
	<div id="footer">
	<!-- Footer start -->
	
		  <center><hr width="75%" class="redline" /></center>
		  
		  <!-- Give a little space -->
          <div style="height:10px"></div>
		  
		  <center><a href="#theTop" class="anchor"  >Back to top</a></center>
		  
		  <br />
          <center>Marriage Encounter Western Cape &reg; is a registered trademark of Marriage Encounter Western Cape, Inc<br />
          &copy; 2009 Marriage Encounter Western Cape, Inc.
          </center>            
        
	<!-- Footer end -->
	</div>    
 
<!-- Div: "Container"  end -->
</div>
 
</form>
 
</body>
 
</html>
 
 
MASTER PAGECODE BEHIND:
 
using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
 
public partial class MimeMaster01 : System.Web.UI.MasterPage
{
    protected void Page_Load(object sender, EventArgs e)
    {
 
    }
 
 
}

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