MSXML The system cannot locate the resource specified

Question

I have a function which uses MSXML to post an Xml document which yesterday started failing despite no change being made to the code. The function is as follows:

Public Function PostFile(ByVal address As String, ByVal data As Byte()) As xmldocument
    Dim xmlHTTP As New MSXML2.XMLHTTP
    Dim response As New XmlDocument

    Dim xmlDoc As New MSXML2.DOMDocument

    Try
        xmlDoc.load(data)
        xmlHTTP.open("post", address, False)
        xmlHTTP.send(xmlDoc)

        If xmlHTTP.responseXML.xml <> String.Empty Then
            response.LoadXml(xmlHTTP.responseXML.xml)
            Return response
        Else
            Dim result As String = "<NO_Response><Error>the post succeeded to " + address + " but there was no responce returned</Error><Hint>Check FireWall Settings</Hint></NO_Response>"
            response.loadxml(result)
            Return response
        End If

    Catch ex As Exception
        'Error logging code removed

    End Try
    Return Nothing
End Function

The Xml document and the address being passed in are both correct - the line which causes the error is xmlHTTP.send(xmlDoc). I have tried this on 2 different machines with the same error each time, and have also tried resinstalling MSXML3, with no success.

The exception thrown is:

    InnerException: Nothing 
    Message:        "The system cannot locate the resource specified. "
    Source:         "msxml3.dll"    
    StackTrace:     "   at MSXML2.XMLHTTPClass.send(Object varBody)    at comms.HTTPHandler.PostFile(String address, Byte[] data) in D:\SCC\Main\Sender\Http.vb:line 42"