Well, I reinstalled it according to Falko's Perfect Debian HowTo (http://www.howtoforge.com/perfect_setup_debian_sarge_p4
) but there were still the same errors.
I had to edit /etc/postfix/sasl/smtpd.conf and alter the saslauth_path to:
and I had to enable chroot in the /etc/postfix/master.cf file:
# service type private unpriv chroot wakeup maxproc command + args
# (yes) (yes) (yes) (never) (100)
smtp inet n - y - - smtpd
Now it works again like a charm