Finally
If RemoteStreamWriter IsNot Nothing Then
   RemoteStreamWriter.Close()  '<---- Hier ist der Fehler ------
   RemoteStreamWriter.Dispose()
End If

Imports System.Net
 
    Public Function FTP_UploadFile(ByVal LocalFile As String, ByVal Servername _
      As String, ByVal Benutzername As String, ByVal Passwort As String, ByVal _
      RemoteFilePath As String, ByVal FTP_mode As Boolean, Optional ByVal _
      Buffer As Integer = 1024) As String
 
        Label1.Text = "Upload auf: " & Servername
        Label2.Text = System.IO.Path.GetFileName(LocalFile) : _
          My.Application.DoEvents()
        Servername = String.Format("{0}/{1}", Servername, _
        System.IO.Path.GetFileName(LocalFile))
        If Servername.IndexOf("ftp://", vbTextCompare) Then Servername = _
        "ftp://" & Servername
 
        Dim requestStream As Stream = Nothing
        Dim fileStream As FileStream = Nothing
        Dim uploadResponse As FtpWebResponse = Nothing
        Dim RemoteStreamWriter As Stream
        Dim LocalStreamReader As BinaryReader
        Try
            Dim uploadRequest As FtpWebRequest = WebRequest.Create(Servername)
            uploadRequest.Method = WebRequestMethods.Ftp.UploadFile
            uploadRequest.Credentials = New NetworkCredential(Benutzername, _
              Passwort)
            uploadRequest.Proxy = Nothing
            uploadRequest.UsePassive = False
 
            Dim iLocalSize As Long = New FileInfo(LocalFile).Length
            ProgressBarAV.Maximum = Int(iLocalSize / 1024) + 2
            ProgressBarAV.Value = 0
            Dim fs As New FileStream(LocalFile, FileMode.Open, FileAccess.Read, _
              FileShare.Read, Buffer)
 
            LocalStreamReader = New BinaryReader(fs)
 
            RemoteStreamWriter = uploadRequest.GetRequestStream
 
            Dim buff As Byte() = New Byte(Buffer - 1) {}
            Dim iBytesRead As Integer = 1
 
            While iBytesRead > 0
 
                iBytesRead = LocalStreamReader.Read(buff, 0, buff.Length)
 
                RemoteStreamWriter.Write(buff, 0, iBytesRead)
                ProgressBarAV.Value = ProgressBarAV.Value + 1
                My.Application.DoEvents()
 
            End While
            fs.Close()
            fs.Dispose()
 
            FTP_UploadFile = "Upload erfolgreich beendet"
 
        Catch ex As UriFormatException
            FTP_UploadFile = ex.Message
        Catch ex As IOException
            FTP_UploadFile = ex.Message
        Catch ex As WebException
            FTP_UploadFile = ex.Message
        Finally
 
            If RemoteStreamWriter IsNot Nothing Then
                RemoteStreamWriter.Close()   '<---- Hier ist der Fehler -----
                RemoteStreamWriter.Dispose()
            End If
 
            If LocalStreamReader IsNot Nothing Then
                LocalStreamReader.Close()
            End If
 
        End Try
 
    End Function

Joachim