Q2.0.15: EFS fails with "500 AUTH not understood"

A typical error: FTP Error: USER request failed; 500 AUTH not understood.

Thanks to giacomo boffi giacomo.boffi@polimi.it who recommends on comp.emacs.xemacs:

tell your ftp client to not attempt AUTH authentication (or do not use FTP servers that don’t understand AUTH)

and notes that you need to add an element (often "-u") to efs-ftp-program-args. Use M-x customize-variable, and verify the needed flag with man ftp or other local documentation.