Access Denied issue when connecting to XERO using oAuth2.0

I am creating an application that will allow a user to send Invoices from a local database application to XERO.

Connection works fine when running on my machine at the office but when I port it to the clients terminal server I get this error.

System.Net.HttpListenerException (0x80004005): Access is denied
   at System.Net.HttpListener.AddAllPrefixes()
   at System.Net.HttpListener.Start()
   at XeroConnectionTest.clsXeroAuth2.VB$StateMachine_6_oauth2.MoveNext() in Q:\XeroConnectionTest\XeroConnectionTest\clsXeroAuth2.vb:line 21

This is the code where it fails


    Private callbackurl As String = "http://localhost:5000/signin/"
    
        Public Async Function oauth2() As Task(Of Boolean)
            Dim redirectUri As String = String.Format(callbackurl)
            Dim listener = New HttpListener()
            listener.Prefixes.Add(redirectUri)
    
            Try
                listener.Start()
                stopListening = False
            Catch hlex As HttpListenerException
                'MsgBox(hlex.ToString)
                Form1.TextBox1.Text = hlex.ToString
                Return False
            End Try

and the error occurs on listener.start() line.

Any help would be gratefully received

Thanks

Frostie