dovecot auth error: Illegal seek

[panetta]

Hi,

I recently configured dovecot to manage auth
for both local and virtual user.
When i login as a virtual user (claudio.panetta) I get the following 
message:

dovecot: auth: Error: 
passwd(claudio.panetta,160.97.62.1,<WLjlnZ9oRACgYT4B>): getpwnam() 
failed: Illegal seek

but login is ok and sending/receiving email is ok,
how can, if possible, I suppress this error message?

In the following my dovecot config:

host-prompt# dovecot -n
# 2.1.7: /etc/dovecot/dovecot.conf
# OS: Linux 3.2.0-5-686-pae i686 Debian 7.11
auth_mechanisms = plain login
auth_username_format = %Ln
listen = *
log_timestamp = "%Y-%m-%d %H:%M:%S "
login_greeting = Server ready.
mail_full_filesystem_access = yes
mail_location = mbox:~/:INBOX=/var/mail/%u:INDEX=/var/index/%u
mail_privileged_group = mail
passdb {
   driver = pam
}
passdb {
   args = scheme=MD5-CRYPT username_format=%n /etc/dovecot/users
   driver = passwd-file
}
protocols = " imap"
service auth {
   unix_listener /var/spool/postfix/private/auth {
     group = postfix
     mode = 0660
     user = postfix
   }
   user = root
}
service imap-login {
   inet_listener imap {
     port = 0
   }
}
ssl_cert = </etc/ssl/certs/chained-cert-9599-ml.mat.unical.it.pem
ssl_key = </etc/ssl/private/csrkey-ml.mat.unical.it.pem
userdb {
   driver = passwd
}
userdb {
   args = username_format=%n /etc/dovecot/users uid=vmail gid=vmail 
home=/var/vmail/%d/%n mail=maildir:/var/vmail/%d/%n/Maildir
   driver = static
}
verbose_proctitle = yes

host-prompt# cat /etc/dovecot/users
claudio.panetta:{MD5-CRYPT}$1$abcdefghijklmnopqrst
ciccio.pasticcio:{MD5-CRYPT}$1$abcdefghijklmnopqrst

Regards,
Claudio