[Solved] ASP.Net UpdatePanel: Invalid postback or callback argument

protected void Page_Load(object sender, EventArgs e)  
{        
    if (!IsPostBack)
    {                     
        drpschool.Items.Insert(0, new ListItem("--Select--", "0"));       
    }
}    
protected void btnsubmit_Click(object sender, EventArgs e) 
{
    con.Open();
    Session["institute"] = drpschool.SelectedItem.Value.ToString();
    Session["year"] = DropDownList1.SelectedItem.Value.ToString();
    con.Close();        Response.Redirect("Delete.aspx");           
}  
protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{               
    if (DropDownList1.SelectedItem.Value == "1st yr")       
    {
        con.Open();
        SqlDataAdapter adp = new SqlDataAdapter("select distinct i_name from GNM1styr_2014_15", con);
        DataSet ds = new DataSet();
        adp.Fill(ds, "GNM1styr_2014_15");        
        con.Close();
        drpschool.Items.Insert(0, new ListItem("--Select--", "0"));
        drpschool.DataTextField = ds.Tables[0].Columns[0].ToString();
        drpschool.DataSource = ds;            
        drpschool.DataBind();                              
    }
    if (DropDownList1.SelectedItem.Value == "2nd yr")        {
        con.Open();
        SqlDataAdapter adp = new SqlDataAdapter("select distinct i_name from GNM2ndyr_2014_15", con);
        DataSet ds = new DataSet();
        adp.Fill(ds, "GNM2ndyr_2014_15");          
        con.Close();
        drpschool.Items.Insert(0, new ListItem("--Select--", "0"));
        drpschool.DataTextField = ds.Tables[0].Columns[0].ToString();
        drpschool.DataSource = ds;         
        drpschool.DataBind();
                }       
    if (DropDownList1.SelectedItem.Value == "3rd yr")
    {            
        con.Open();
        SqlDataAdapter adp = new SqlDataAdapter("select distinct i_name from GNM3rdyr_2014_15", con);
        DataSet ds = new DataSet();
        adp.Fill(ds, "GNM3rdyr_2014_15");         
        con.Close();
        drpschool.Items.Insert(0, new ListItem("--Select--", "0"));
        drpschool.DataTextField = ds.Tables[0].Columns[0].ToString();
        drpschool.DataSource = ds;           
        drpschool.DataBind();    
    }
}

<form id="form1" runat="server">
<div>
    <asp:UpdatePanel ID="UpdatePanel2" runat="server">
        <ContentTemplate>
            <asp:DropDownList ID="DropDownList1" runat="server" Width="450px" AppendDataBoundItems="true"
                AutoPostBack="True" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged">
                <asp:ListItem>Select</asp:ListItem>
                <asp:ListItem>1st yr</asp:ListItem>
                <asp:ListItem>2nd yr</asp:ListItem>
                <asp:ListItem>3rd yr</asp:ListItem>
                <asp:ListItem></asp:ListItem>
            </asp:DropDownList>
            <asp:DropDownList ID="drpschool" Width="450px" runat="server">
            </asp:DropDownList>
        </ContentTemplate>
    </asp:UpdatePanel>
    <asp:UpdateProgress ID="UpdateProgress1" runat="server" AssociatedUpdatePanelID="UpdatePanel2">
        <ProgressTemplate>
            <div>
                loading.......</div>
        </ProgressTemplate>
    </asp:UpdateProgress>
    </td> </tr>
    <tr>
        <td align="left" style="width: 200px; height: 40px">
        </td>
        <td align="left" style="width: 300px; height: 40px">
            <asp:Button ID="btnsubmit" runat="server" BackColor="#8080FF" BorderColor="#8080FF"
                CssClass="panel" Font-Bold="True" ForeColor="White" Height="31px" OnClick="btnsubmit_Click"
                Text="Submit" Width="100px" />
        </td>
    </tr>
    <tr>
        <td align="left" style="width: 200px; height: 20px">
        </td>
        <td align="left" style="width: 300px; height: 20px">
            &nbsp;
        </td>
    </tr>
</table>
</form>

 hello sir i have two dropdown inside a updatepael when i select one dropown th other gets update and on button click redirect o another page but when i move back to he same page it displays error invalid postback or callbck ,if datais validuse clietscriptmanager.registerfor eventvalidaion method