I have a web client application built on .Net Frameworks 2.0 that connects to
a web service on IIS 6.0 via SSL and using X509 Client Certificates.  At some
locations (it works at others), I get the following error.  Below is my code.
I've also used System.Net.Tracing and have the traffic generated as a result
of the web request if anyone wants to view it.

System.Net.WebException: There was an error downloading
'https://web-site-url/web-service.asmx'. ---> System.Net.WebException: The
underlying connection was closed: An unexpected error occurred on a send.
---> System.IO.IOException: The handshake failed due to an unexpected packet
format.
  at System.Net.Security.SslState.StartReadFrame(Byte[] buffer, Int32
readBytes, AsyncProtocolRequest asyncRequest)
... remainder ommitted for brevity.

THE CODE:

' Connect to Web Service
Dim metwebProxy As New CalAuthWs
Dim userProxy As IWebProxy

' Set certificate policy to ignore metweb certificate error notifications
ServicePointManager.ServerCertificateValidationCallback =
g_certificateValidationCallback

metwebProxy.PreAuthenticate = True

' Set Network Proxy Server depending on user selections
If String.Compare(My.Settings.UserProxy, m_PROXY_DEFAULT, True) = 0 Then
....userProxy = WebRequest.DefaultWebProxy
....If Not userProxy Is Nothing Then
........If My.Settings.UseDefaultCredentials Then
............userProxy.Credentials = CredentialCache.DefaultCredentials
........End If
........metwebProxy.Proxy = userProxy
....End If
ElseIf String.Compare(My.Settings.UserProxy, m_PROXY_USER_DEFINED, True) = 0
Then
....' User has manual proxy server settings
....userProxy = Utilities.GetUserProxyServer
....If Not userProxy Is Nothing Then metwebProxy.Proxy = userProxy
End If

' Add client-side certificate
metwebProxy.ClientCertificates.Add(x509In)

' Test Connection
If metwebProxy.TestLogin(certificateSubject, ApplicationSource.AFCAV) Then
....Return metwebProxy
Else
....' Web Service indicates system is down
....Dim status As String = metwebProxy.ServerStatus
....DisplayMessage(True, "Access Denied: WebAFCAV is currently not
available.  Status: " & status)
....Return Nothing
End If