Google didn’t help much or at least I didn’t find anything useful. After some trial-and-error research with the configuration switches I found out that the ‘-s’ switch was responsible for the problems. Here’s what the documentation says about it:

‘-s’: The “waReZ protection”. Don’t allow anonymous users to download files owned by “ftp” (generally, files uploaded by other anonymous users) . So that uploads have to be validated by a system administrator (chown to another user) before being available for download.

Obviously the server was looking for a system-account (ftp) that didn’t exist, which is hardly suprising as I use other accounts for ftp-ing. I didn’t find any hints on configuring the account-id Pure-FTPd expects. This anti-warez thingy being just a switch, there seems to be no solution apart from creating the ‘ftp’ account. Thus I finally disabled the anti-warez feature. Maybe someone has the same problem and finds this post useful.