ok guys, seems like my post was an ignition spark.
to extinguish the fire, I am making an update on this problem.
the proftpd default installation works as it supposed to be. I have reinstalled the server, reset user's password and voila it works. sofar I have no idea what could be wrong...
regarding the discussion: I moved to proftpd some years ago due to its advertised improved security. Then I did not use it and follow the topic for several years. I have trusted the developers that it would be a priority for this project, that is why I decided to use it recently again (and as some of you already pointed out, because the configuration was quite intuitive for me).
point for you guys for warning me to update to the latest realease.