I guess it was a little late while I was trying.
I had the impression WebRequest.Create would just open a connection to the FTPServer,
but it also has to point to the file to which it should be uploaded.
I should have noticed ealier since my code doesnt tell
the FTP Server to which file it should upload. 
My new code:
[code=vb] Public Shared Function TextUpload(ByVal Host As String, ByVal Port As UInt16, ByVal UserName As String, ByVal Password As String, ByVal SourceStr As String, ByVal FileName As String) As Boolean
Dim FTPCon As FtpWebRequest
FTPCon = WebRequest.Create(Host & ":" & Port & "/" & FileName)
FTPCon.Method = WebRequestMethods.Ftp.UploadFile
FTPCon.Credentials = New NetworkCredential(UserName, Password)
Dim FileContent() As Byte = Encoding.ASCII.GetBytes(SourceStr)
FTPCon.ContentLength = FileContent.Length
Try
Dim ReqStream As Stream = FTPCon.GetRequestStream
ReqStream.Write(FileContent, 0, FileContent.Length)
ReqStream.Close()
Dim FTPRes As FtpWebResponse = FTPCon.GetResponse
Return True
Catch ex As Exception
Return False
End Try
End Function
[/code]