[Dovecot] Dovecot deliver LDA problem

[Alex]

Hello folks,
I was trying to set up Dovecot deliver LDA instead of Postfix default
virtual LDA

In Postfix main.cf I specified:
virtual_transport = dovecot
dovecot_destination_recipient_limit = 1

In Postfix master.cf  I entered:
dovecot   unix  -       n       n       -       -       pipe
        user=mailer flags=DRhu
argv=/usr/local/mail/libexec/dovecot/deliver -f ${sender} -d ${recipient}

In Dovecot dovecot.conf  I have the following entries:
passdb sql {
    args = /etc/dovecot/dovecot-sql.conf
}

userdb prefetch {
}

userdb sql {
    args = /etc/dovecot/dovecot-sql.conf
}

In Dovecot dovecot-sql.conf  I have this:
user_query = SELECT maildir as mail FROM dovecot_email_maps WHERE email
= '%u'
password_query = SELECT user, password, userdb_uid, userdb_gid,
userdb_home, userdb_mail FROM dovecot_user_maps WHERE user = '%n'

After reloading postfix and restarting dovecot I'm getting this type of
warnings in dovecot.err log:

May 04 10:43:10 deliver(some_address at some_domain.com): Error: userdb
lookup: connect(/var/run/dovecot//auth-master) failed: No such file or
directory

Could somebody please help me to resolve this mistery.
What am I missing ether in configuration file or somewhere else?

Many thanks in advance,

Alex