Well, I've gotten this error for a few times when I tried to run duplicity/ftplicity.

cp:/backup# FTP_PASSWORD=********* duplicity /backup/current ftp://*********@*********/test
Traceback (most recent call last):
  File "/usr/bin/duplicity", line 373, in ?
    if __name__ == "__main__": main()
  File "/usr/bin/duplicity", line 349, in main
    globals.archive_dir).set_values()
  File "/usr/lib/python2.4/site-packages/duplicity/collections.py", line 406, in set_values
    backend_filename_list = self.backend.list()
  File "/usr/lib/python2.4/site-packages/duplicity/backends.py", line 365, in list
    try: return self.error_wrap('nlst')
  File "/usr/lib/python2.4/site-packages/duplicity/backends.py", line 335, in error_wrap
    except ftplib.all_errors, e: raise BackendException(e)
duplicity.backends.BackendException: 226 Transfer complete.
cp:/backup#

Well, I don't know the reason why this happens but I know how to prevent this. This happens if you try to run duplicity or ftplicity into a backup folder that is EMTPY. In that case above, the folder /test on the server was empty. I just needed to create one file there and afterwards it runs fine.