Uploading and downloading files with FTP
You aren't doing proper error handling.  For example, changing the directory could fail due to a permission problem.  In addition to pre-screening each commands, you need to check the status returned by the server.  In RFC 959, linked from the ftplib documentation, you'll see that status codes ≥ 400 indicate an error.  (You'll need to include an English-to-German translation table for the errors listed in Section 4.2.2 of the RFC.  Unfortunately, the errors aren't very specific as to the exact cause.)