[Dovecot] Authentication failure messages in logs

[Nicolas Mora]

Hello,

I'm currently running dovecot on a debian stable and every day, I see 
this message dozens of time in my logs :

Mar 16 11:27:57 hector dovecot-auth: pam_unix(dovecot:auth): 
authentication failure; logname= uid=0 euid=0 tty=dovecot ruser=nicolas 
rhost=72.53.129.223  user=nicolas

on the list archive, this message for example tells to comment out the 
pam section.
http://www.dovecot.org/list/dovecot/2008-July/031966.html
Except that my authentication needs pam...

Is there any way to solve this without migrating to virtual users ?

Thanks in advance

Here is my dovecot -n output :

# 1.2.15: /etc/dovecot/dovecot.conf
# OS: Linux 3.2.13-grsec-xxxx-grs-ipv6-64 x86_64 Debian 6.0.7
protocols: imap imaps managesieve sieve
ssl_listen(default): *:993
ssl_listen(imap): *:993
ssl_listen(managesieve):
ssl_cert_file: /etc/ssl/localcerts/dovecot.pem
ssl_key_file: /etc/ssl/localcerts/dovecot.key
login_dir: /var/run/dovecot/login
login_executable(default): /usr/lib/dovecot/imap-login
login_executable(imap): /usr/lib/dovecot/imap-login
login_executable(managesieve): /usr/lib/dovecot/managesieve-login
mail_privileged_group: mail
mail_location: maildir:~/Maildir
mbox_write_locks: fcntl dotlock
mail_executable(default): /usr/lib/dovecot/imap
mail_executable(imap): /usr/lib/dovecot/imap
mail_executable(managesieve): /usr/lib/dovecot/managesieve
mail_plugin_dir(default): /usr/lib/dovecot/modules/imap
mail_plugin_dir(imap): /usr/lib/dovecot/modules/imap
mail_plugin_dir(managesieve): /usr/lib/dovecot/modules/managesieve
lda:
   postmaster_address: postmaster at babelouest.org
   mail_plugins: sieve
   mail_plugin_dir: /usr/lib/dovecot/modules/lda
   sieve_dir: ~/
auth default:
   mechanisms: plain login
   passdb:
     driver: pam
     args: dovecot
   userdb:
     driver: passwd
   socket:
     type: listen
     client:
       path: /var/spool/postfix/private/auth
       mode: 432
       user: postfix
       group: postfix
     master:
       path: /var/run/dovecot/auth-master
       mode: 384
plugin:
   sieve: ~/.dovecot.sieve