sieve - flags : delivery failure

[Manuele]

I am building a new mail server with:
postfix, cyrus, mysql on a slackware linux box
everything so far worked pretty well until I tried to
use the addflag / removeflag extensions to sieve...

in fact, as soon as a sieve script is triggered and tries to set/remove
flags deliver hangs (temporary failure) and mail is being queued again by
postfix.

imapd.log shows:
seen_db: user manu opened /var/imap/user/m/manu.seen
but delivery is never finished...

/var/imap and mail dirs are all on a reiserfs filesystem...
can anyone help on that?
TIA