Hello Brano,
the fact that you don't have do_it installed is your problem.
Every time you boot your NAS proftpd.conf is restored from flash. That is normal behaviour of the 2.3.2.IB.2.RS.1 firmware release.
Therefore I created do_it. Here is a
thread describing do_it although it is written in German. You could try Google translation.
Download do_it.tgz and copy the file to (/mnt/IDE1/) public/applications/new_software as it is described in the documentation written by Raidsonic.
(Same procedure as for SSH server package)
Regards,
HWguru