Authorization error (401) when loading treeview

I am having a problem loading a tree view web control.
This problem does not occur on my development machine but
occurs when testing on a QA server.

We can load dropdown lists with data on the same pages.
When attempting to load the treeview, a trace of the call
to the database reveals no database security issue and
the data is retrieved.
I believe that the group permission and impersonation is
setup correctly on the test machine (as we can access the
application, the pages within the applicationand some
data.
The error log stack trace (see below) indicates that
there is a final HttpWebRequest check (CheckFinalStatus)
that is the point of failure. MSDN not too helpful in
terms of anything I might have ommitted.

Here is the Error log:

The remote server returned an error: (401) Unauthorized.

STACK TRACE:

at System.Net.HttpWebRequest.CheckFinalStatus()
at System.Net.HttpWebRequest.EndGetResponse
(IAsyncResult asyncResult)
at System.Net.HttpWebRequest.GetResponse()
at System.Xml.XmlDownloadManager.GetNonFileStream(Uri
uri, ICredentials credentials)
at System.Xml.XmlDownloadManager.GetStream(Uri uri,
ICredentials credentials)
at System.Xml.XmlUrlResolver.GetEntity(Uri
absoluteUri, String role, Type ofObjectToReturn)
at System.Xml.XmlTextReader.CreateScanner()
at System.Xml.XmlTextReader.Init()
at System.Xml.XmlTextReader.Read()
at
System.Xml.XmlValidatingReader.ReadWithCollectText Token()
at System.Xml.XmlValidatingReader.Read()
at System.Xml.XPath.XPathDocument.Load(XmlReader
reader)
at System.Xml.Xsl.Compiler.LoadDocument(XmlTextReader
reader)
at System.Xml.Xsl.XslTransform.Load(String url,
XmlResolver resolver)
at System.Xml.Xsl.XslTransform.Load(String url)
at
Microsoft.Web.UI.WebControls.TreeView.GetXmlReader FromXPat
hDoc(XPathDocument xmldoc, String TreeNodeXsltSrc)
at
Microsoft.Web.UI.WebControls.TreeView.GetXmlReader FromStri
ng(String TreeNodeSrc, String TreeNodeXsltSrc)
at Microsoft.Web.UI.WebControls.TreeView.ReadXmlSrc
(String TreeNodeSrc, String TreeNodeXsltSrc, String
strOuter)
at
Microsoft.Web.UI.WebControls.TreeView.ReadTreeNode XmlSrc()
at Microsoft.Web.UI.WebControls.TreeView.DataBind()
at CMSDataViewer.OrgUnit.OrgUnit.RefreshTree()
at
CMSDataViewer.OrgUnit.OrgUnit.cboGrouping_Selected IndexCha
nged(Object sender, EventArgs e)
at
System.Web.UI.WebControls.ListControl.OnSelectedIn dexChang
ed(EventArgs e)
at
System.Web.UI.WebControls.ListBox.System.Web.UI.IP ostBackD
ataHandler.RaisePostDataChangedEvent()
at System.Web.UI.Page.RaiseChangedEvents()
at System.Web.UI.Page.ProcessRequestMain()

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Hopefully someone can point me in the right direction
with this.

Cheers.

Thhanks in advance. Grant S