Visual Basic.NET - winService con API

Buenas Tardes
Porfavor su ayudaa, soy nueva enviando archivos a un url, no se porque me muestra este error, estoy revisando el c{odigo varios días y ya no se que más podría ser

"Error en el servidor remoto: (500) Error interno del servidor" y se detiene en esta linea
" Using response As HttpWebResponse = CType(request.GetResponse(), HttpWebResponse)"


este es el código:

Public Function Request_PostDocument(ByVal nombrefichero As String, ByVal fichero As String,
ByVal Accesso As String) As String
Dim retVal As String = ""

Dim FilePath As String = pathDefaultProcesados & "\" & nombrefichero
Dim request As HttpWebRequest = CType(WebRequest.Create(urlDocum), HttpWebRequest)
Dim encodedCadena As String = Nothing

Dim cadenaByte As Byte() = New Byte((FilePath).Length - 1) {}
'----------------Headers----------------------------------
request.Headers("Authorization") = "Bearer " + Accesso
'-----------Body------------------------------------
request.Method = "POST"
request.ContentType = "multipart/form-data"
Dim postencoding As ASCIIEncoding = New ASCIIEncoding()
Dim postdata As String = ("Content-Disposition: file; name=" + fichero + "&filename=" + FilePath)
Dim data() As Byte = postencoding.GetBytes(postdata)
request.ContentLength = data.Length
request.ContentLength = cadenaByte.Length

Dim requestStream As Stream = request.GetRequestStream()
requestStream.Write(cadenaByte, 0, cadenaByte.Length)
requestStream.Close()

Using response As HttpWebResponse = CType(request.GetResponse(), HttpWebResponse)
Using reader As StreamReader = New StreamReader(response.GetResponseStream())
Return reader.ReadToEnd()
End Using
MsgBox(CType(response, HttpWebResponse).StatusDescription)
End Using