So then, after all that effort in trying to get the FTP server on my linux box to allow real users to connect to it, I finaily find the problem. wu-ftpd was broken. 'cause after doing an
apt-get upgrade
it upgraded wu-ftpd and all seems to work now... grrrrrrr.
After further research, it wasn't really wu-ftpd's fault. It's the PAM authentication thingy me jig, that wanted the ftpusers file to be a proper file and not a symbolicly linked one. How many people are still with me? :)