i've just figured out what's happening. It seems that the first configuration of pure-ftpd-mysql doesn't work as expected: the generated login credeantials in /etc/pure-ftpd/db/mysql.conf are incorrect. OK, so i've changed the credentials to user ispconfig and real password... and here comes the strange part: restarting pure-ftpd with pure-ftpd-control restart doesn't stop the daemon properly (neither with pure-ftpd-control stop), so a "fake" restart occurs (with no new real credentials). I've killed the daemon, started it again... and it works.
Restarting the server should work, but I haven't tried.
EDIT: the error occurs when accesing ftp, because pure-ftpd tries to read db with incorrect credentials.
Hope it helps,
Last edited by astuto; 28th October 2010 at 19:33.