[Dovecot] Why can NOT login as root

[jana1972 at centrum.cz]

Hi i use Dovecot and it works ok for users except for root user.
It is impossible to login as  root 
Here is a log


Sep 10 10:15:44 auth: Debug: Loading modules from directory: 
/usr/local/lib/dovecot/auth
Sep 10 10:15:44 auth: Debug: auth client connected (pid=18077)
Sep 10 10:15:44 auth: Debug: client in: AUTH	1	PLAIN	service=imap	
secured	lip=212.97.133.82	rip=212.97.133.82	lport=143	
rport=34645	resp=<hidden>
Sep 10 10:15:44 auth: Debug: Loading modules from directory: 
/usr/local/lib/dovecot/auth
Sep 10 10:15:44 auth: Debug: pam(root,212.97.133.82): lookup service=dovecot
Sep 10 10:15:44 auth: Debug: pam(root,212.97.133.82): #1/1 style=1 
msg=Password: 
Sep 10 10:15:44 auth: Debug: client out: OK	1	user=root
Sep 10 10:15:44 auth: Debug: master in: REQUEST	2889482241	18077	1	
3500554cf70742dfc0515671c7671bbd
Sep 10 10:15:44 auth: Debug: passwd(root,212.97.133.82): lookup
Sep 10 10:15:44 auth: Debug: master out: USER	2889482241	root	
system_groups_user=root	uid=0	gid=0	home=/root
Sep 10 10:15:44 imap-login: Info: Login: user=<root>, method=PLAIN, 
rip=212.97.133.82, lip=212.97.133.82, mpid=18082, secured
Sep 10 10:15:44 imap: Error: user root: Invalid settings in userdb: userdb 
returned 0 as uid
Sep 10 10:15:44 imap: Error: Invalid user settings. Refer to server log for 
more information.


Can anyone help?
Thanks
LL.