Updating Active Directory from Web Application Error

I am receiving an error a web based application that allows corporate intranet users to update their active directory details (phone numbers, etc).

The web application is hosted on IIS6 running Windows Server 2003 (SP1). The IIS website is using NTLM Authentication and the website has integrated security enabled. The IIS application pool runs using the “Network Service” account.

The web.config contains the following elements

<LdapConfigurations server="xxx.internal" root="OU=Staff Accounts,DC=xxx,DC=internal" domain="xxx" />
<identify impersonate=”true” />

Active Directory delegation is not needed as the following C# (.NET 3.5) code should pass on the correct impersonation details (including security token) onto Active Directory.

public void UpdateData(string bus, string bus2, string fax, string home, string home2, string mob, string pager, string notes)
{
    WindowsIdentity windId = (WindowsIdentity)HttpContext.Current.User.Identity;
    WindowsImpersonationContext ctx = null;

    try
    {
        ctx = windId.Impersonate();

        DirectorySearcher ds = new DirectorySearcher();
        DirectoryEntry de = new DirectoryEntry();

        ds.Filter = m_LdapUserFilter;

        // i think this is the line causing the error
        de.Path = ds.FindOne().Path;

        this.AssignPropertyValue(bus, ADProperties.Business, ref de);
        this.AssignPropertyValue(bus2, ADProperties.Business2, ref de);
        this.AssignPropertyValue(fax, ADProperties.Fax, ref de);
        this.AssignPropertyValue(home, ADProperties.Home, ref de);
        this.AssignPropertyValue(home2, ADProperties.Home2, ref de);
        this.AssignPropertyValue(mob, ADProperties.Mobile, ref de);
        this.AssignPropertyValue(pager, ADProperties.Pager, ref de);
        this.AssignPropertyValue(notes, ADProperties.Notes, ref de);

        // this may also be causing the error?
        de.CommitChanges();
    }
    finally
    {
        if (ctx != null) 
        {
            ctx.Undo();
        }
    }
}

private void AssignPropertyValue(string number, string propertyName, ref  DirectoryEntry de)
{
    if (number.Length == 0 && de.Properties[propertyName].Value != null)
    {
        de.Properties[propertyName].Remove(de.Properties[propertyName].Value);
    }
    else if (number.Length != 0)
    {
        de.Properties[propertyName].Value = number;
    }
}

User details can be retrieved from Active Directory without a problem however the issue arises when updating the users AD details. The following exception message is displayed.

 System.Runtime.InteropServices.COMException (0x80072020): An operations error occurred. 
       at System.DirectoryServices.DirectoryEntry.Bind(Boolean throwIfFail) 
       at System.DirectoryServices.DirectoryEntry.Bind() 
       at System.DirectoryServices.DirectoryEntry.get_AdsObject() 
       at System.DirectoryServices.DirectorySearcher.FindAll(Boolean findMoreThanOne) 
       at System.DirectoryServices.DirectorySearcher.FindOne()    
       at xxx.UpdateData(String bus, String bus2, String fax, String home, String home2, String mob, String pager, String notes) 
       at xxx._Default.btnUpdate_Click(Object sender, EventArgs e)

The code works fine in our development domain but not in our production domain. Can anyone please assist in helping resolving this problem?

This is more than likely a permissions problem - there are numerous articles regards impersonation and delegation and the vagaries thereof here: http://support.microsoft.com/default.aspx?scid=kb;en-us;329986 and here: http://support.microsoft.com/default.aspx?scid=kb;en-us;810572.