Error trying to write to XML file

Could someone give a clue as to what is wrong with my code? It is simply a textbox, a submit button, and it writes to an XML file. I get the following error: 

Server Error in '/' Application. -------------------------------------------------------------------------------- The resource cannot be found. Description: HTTP 404. The resource you are looking for (or one of its dependencies) could have been removed, had its name changed, or is temporarily unavailable. Please review the following URL and make sure that it is spelled correctly. Requested URL: /CustomError.aspx

<%@ import Namespace="System.Data" %>
<script runat="server">
    Private Sub Page_Load(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles MyBase.Load
        If Not IsPostBack Then
            Using ds As New D ...