530 User guest may not use FTP.After much googling, a French website provided the answer: sharing-only users (=without their own home directory) have the shell /usr/bin/false. Users with this shell can only log on an FTP server if it is listed in /etc/shells (which it isn't).
FTP for sharing files on Mac OS Leopard
I've decided to upgrade from sneakernet (=USB stick) to something more comfortable when sharing files with my brother, namely FTP. This is relatively easy to configure: Enable FTP sharing, create a new sharing-only user and designate a shared folder. But afterwards I kept getting an error: