Visual basic - probleme connection serveur ftp

Salut,

Je me connecte a mon serveur ftp grace a la function:

Code:

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Public Sub UploadFile(ByVal _FileName As String, ByVal _UploadPath As String, ByVal _FTPUser As String, ByVal _FTPPass As String)
 
        Dim _FileInfo As New System.IO.FileInfo(_FileName)
 
 
 
        ' Create FtpWebRequest object from the Uri provided
 
        Dim _FtpWebRequest As System.Net.FtpWebRequest = CType(System.Net.FtpWebRequest.Create(New Uri(_UploadPath)), System.Net.FtpWebRequest)
 
 
 
        ' Provide the WebPermission Credintials
 
        _FtpWebRequest.Credentials = New System.Net.NetworkCredential(_FTPUser, _FTPPass)
 
 
 
        ' By default KeepAlive is true, where the control connection is not closed
 
        ' after a command is executed.
 
        _FtpWebRequest.KeepAlive = False
 
 
 
        ' set timeout for 20 seconds
 
        _FtpWebRequest.Timeout = 20000
 
 
 
        ' Specify the command to be executed.
 
        _FtpWebRequest.Method = System.Net.WebRequestMethods.Ftp.UploadFile
 
 
 
        ' Specify the data transfer type.
 
        _FtpWebRequest.UseBinary = True
 
 
 
        ' Notify the server about the size of the uploaded file
 
        _FtpWebRequest.ContentLength = _FileInfo.Length
 
 
 
        ' The buffer size is set to 2kb
 
        Dim buffLength As Integer = 2048
 
        Dim buff(buffLength - 1) As Byte
 
 
 
        ' Opens a file stream (System.IO.FileStream) to read the file to be uploaded
 
        Dim _FileStream As System.IO.FileStream = _FileInfo.OpenRead()
 
 
 
        Try
 
            ' Stream to which the file to be upload is written
 
            Dim _Stream As System.IO.Stream = _FtpWebRequest.GetRequestStream()
 
 
 
            ' Read from the file stream 2kb at a time
 
            Dim contentLen As Integer = _FileStream.Read(buff, 0, buffLength)
 
 
 
            ' Till Stream content ends
 
            Do While contentLen <> 0
 
                ' Write Content from the file stream to the FTP Upload Stream
 
                _Stream.Write(buff, 0, contentLen)
 
                contentLen = _FileStream.Read(buff, 0, buffLength)
 
            Loop
 
 
 
            ' Close the file stream and the Request Stream
 
            _Stream.Close()
 
            _Stream.Dispose()
 
            _FileStream.Close()
 
            _FileStream.Dispose()
 
        Catch ex As Exception
 
            MsgBox("error")
 
        End Try
 
    End Sub

Ca fonctionne sur certains pc et dautres non, et je n'ai malheuresement aucune informations sur les pc sur lesquels la connction a echoue.
Quelles pourraient etre les causes du disfonctionnement ?