GetRequestStream causes The connection was closed unexpectedly - vb.net

Question

i have a vb.net console application that logged into a website (POST form) by using Webclient:
Dim responsebytes = myWebClient.UploadValues("https:!!xxx.com/mysession/create", "POST", myNameValueCollection)
Last friday this suddenly stopped working, it worked without a problem for about 2-3 years. With Fiddler I got a HTTP 504 error but without Fiddler I got the error message:
The underlying connection was closed: The connection was closed unexpectedly.
I assume that something on the server-side has changed, but I have no influence on that. It's a commercial website, where I want to login automatically on my account to fetch some data.
As Fiddler can't help me much further I decided to built a basic HttpWebRequest example to rule out it was caused by the WebClient.
The example does:
navigate to the homepage of the company and read out an securityToken (this goes ok!)
post the securityToken + username + password to get logged in.
Public Class Form1
Const ConnectURL = "https:!!member.company.com/homepage/index"
Const LoginURL = "https:!!member.company.com/account/logn"
Private Function RegularPage(ByVal URL As String, ByVal CookieJar As CookieContainer) As String
Dim reader As StreamReader
Dim Request As HttpWebRequest = HttpWebRequest.Create(URL)
Request.UserAgent = "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/46.0.2490.80 Safari/537.36"
Request.AllowAutoRedirect = False
Request.CookieContainer = CookieJar
Dim Response As HttpWebResponse = Request.GetResponse()
reader = New StreamReader(Response.GetResponseStream())
Return reader.ReadToEnd()
reader.Close()
Response.Close()
End Function
Private Function LogonPage(ByVal URL As String, ByRef CookieJar As CookieContainer, ByVal PostData As String) As String
Dim reader As StreamReader
Dim Request As HttpWebRequest = HttpWebRequest.Create(URL)
Request.UserAgent = "Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/46.0.2490.80 Safari/537.36"
Request.CookieContainer = CookieJar
Request.AllowAutoRedirect = False
Request.ContentType = "application/x-www-form-urlencoded"
Request.Method = "POST"
Request.ContentLength = PostData.Length
Dim requestStream As Stream = Request.GetRequestStream()
Dim postBytes As Byte() = Encoding.ASCII.GetBytes(PostData)
requestStream.Write(postBytes, 0, postBytes.Length)
requestStream.Close()
Dim Response As HttpWebResponse = Request.GetResponse()
For Each tempCookie In Response.Cookies
CookieJar.Add(tempCookie)
Next
reader = New StreamReader(Response.GetResponseStream())
Return reader.ReadToEnd()
reader.Close()
Response.Close()
End Function
Private Sub Button1_Click(sender As Object, e As EventArgs) Handles Button1.Click
Dim CookieJar As New CookieContainer
Dim PostData As String
Try
Dim homePage As String = (RegularPage(ConnectURL, CookieJar))
Dim securityToken = homePage.Substring(homePage.IndexOf("securityToken") + 22, 36) 'momenteel 36 characters lang
PostData = "securityToken=" + securityToken + "&accountId=123456789&password=mypassword"
MsgBox(PostData)
Dim accountPage As String = (LogonPage(LoginURL, CookieJar, PostData))
Catch ex As Exception
MsgBox(ex.Message.ToString)
End Try
End Sub
End Class
This line causes the connection to be closed:
Dim requestStream As Stream = Request.GetRequestStream()
Is it possible that this company doesnt like the automated login and somehow notices that a application is used for logging in? How can I debug this? Fiddler doesn't seem to work. Is my only option WireShark as this seems kind of difficult to me.
Also is it weird that the connection is already is closed before I do the Post?
Are there other languages I can program this "easily" to rule out it's VB.net / .NET problem?

Answer 1

Have you attempted to capture the request using something like your browser's networking tools?
The auth process may have changed. Could even be some name or post data changes.

Answer 2

I got this fixed by:
double checking all the headers to be sent when using a browser
made sure all those headers where sent by the VB.NET application.
Not sure which one did the trick, but just always make sure you replicate all the headers that the browser would sent!