ASP.NET File download over HTTPS failure

Stuck on trying to download a file over https, works fine across http but get the message in firefox - 'FILENAME could not be saved because the source file could not be read' and 'Failed - Network Error' in Chrome.

Web application sits behind Cisco firewall in which i have opened all the ports and still no joy. Flomoxed.

    Dim file As System.IO.FileInfo = New System.IO.FileInfo(FullFilePath) '-- if the file exists on the server
    If file.Exists Then 'set appropriate headers

        Response.ClearContent()
        Response.ClearHeaders()

        Dim iStream As System.IO.Stream = Nothing
        ' Buffer to read 10K bytes in chunk:
        Dim buffer(10000) As Byte
        ' Length of the file:
        Dim length As Integer
        ' Total bytes to read:
        Dim dataToRead As Long

        Try
            ' Open the file.
            iStream = New System.IO.FileStream(FullFilePath, System.IO.FileMode.Open,
                                                   IO.FileAccess.Read, IO.FileShare.Read)

            ' Total bytes to read:
            dataToRead = iStream.Length

            'Response.ContentType = "application/octet-stream"
            Response.ContentType = "application/pdf"
            'Response.ContentType = WebApp.Common.Functions.ReturnContentType(file.Extension.ToLower())
            Response.AddHeader("Content-Disposition", "attachment; filename=""" & file.Name & """")

            ' Read the bytes.
            While dataToRead > 0
                ' Verify that the client is connected.
                If Response.IsClientConnected Then
                    ' Read the data in buffer
                    length = iStream.Read(buffer, 0, 10000)

                    ' Write the data to the current output stream.
                    Response.OutputStream.Write(buffer, 0, length)

                    ' Flush the data to the HTML output.
                    Response.Flush()

                    ReDim buffer(10000) ' Clear the buffer
                    dataToRead = dataToRead - length
                Else
                    'prevent infinite loop if user disconnects
                    dataToRead = -1
                End If
            End While

        Catch ex As Exception
            ' Trap the error, if any.
            Response.Write("Error : " & ex.Message)
        Finally
            If iStream IsNot Nothing Then
                ' Close the file.
                iStream.Close()
            End If
            Response.Close()
        End Try

        Response.End() 'if file does not exist
    Else
        Response.Write("This file does not exist.")
    End If 'nothing in the URL as HTTP GET