HTTP Response 500 - Using VB.Net IPN Listener

[Eggybread123]

I have a website which is written in VB.Net/Asp.Net and it has an IPN Listener (Code Below). It has worked for the last 6 months but now it has stopped and shows that transactions are 'Retrying' in PayPal's IPN Listener History. I don't know what to change or do if anyone can please help?

Imports System.Net
Imports System.IO
Imports System.Web.Util
Imports System.Data.Common

Partial Class IPNListener
    Inherits System.Web.UI.Page

    Protected Sub Page_Load(sender As Object, e As System.EventArgs) Handles Me.Load
        'Post back to either sandbox or live
        'Dim strURL As String = "https://www.sandbox.paypal.com/cgi-bin/webscr"
        Dim strURL As String = "https://www.paypal.com/cgi-bin/webscr"
        Dim req As HttpWebRequest = CType(WebRequest.Create(strURL), HttpWebRequest)

        'Set values for the request back        req.Method = "POST"        req.ContentType = "application/x-www-form-urlencoded"        req.Proxy = New WebProxy("ntproxyus.lxa.perfora.net", 3128)

        Dim Param() As Byte = Request.BinaryRead(HttpContext.Current.Request.ContentLength)
        Dim strRequest As String = Encoding.ASCII.GetString(Param)        strRequest = strRequest + "&cmd=_notify-validate"        req.ContentLength = strRequest.Length

        'Send the request to PayPal and get the response
        Dim streamOut As StreamWriter = New StreamWriter(req.GetRequestStream(), Encoding.ASCII)        streamOut.Write(strRequest)        streamOut.Close()
        Dim streamIn As StreamReader = New StreamReader(req.GetResponse().GetResponseStream())
        Dim strResponse As String = streamIn.ReadToEnd()        streamIn.Close()

        Dim qscoll As NameValueCollection = HttpUtility.ParseQueryString(strRequest)

        'Insert the paypal response
        Dim order As New orders
        order.InsertPaypalResponse(qscoll("txn_id"), qscoll("custom"), strRequest)

        If strResponse = "VERIFIED" Then              order.UpdateOrderFromPaypal(qscoll("custom"), qscoll("txn_id"), qscoll("payment_status"))
        ElseIf strResponse = "INVALID" Then
            'log for manual investigation            order.UpdateOrderFromPaypal(qscoll("custom"), qscoll("txn_id"), qscoll("payment_status"))
        Else
            'Response wasn't VERIFIED or INVALID, log for manual investigation            order.UpdateOrderFromPaypal(qscoll("custom"), qscoll("txn_id"), qscoll("payment_status"))
        End If
    End Sub
End Class