[Dovecot] Dovecot v1.1.alpha4 problem

[Vladislav Malyshkin]

Dovecot v1.1.alpha4 imap after some time stops responding.
It can not be restarted as
/etc/init.d/dovecot restart

killall imap
also does not help

killall -9 imap
and then restart does help

in log I see message (repeted periodically) that
Sep 12 09:50:02 localhost postfix/local[22533]: C7057753B6: 
to=<username at domain>
, relay=local, delay=5195, delays=5156/0.01/0/39, dsn=4.2.0, status=deferr
ed (cannot update mailbox /home/username/Mailbox for user username. 
unable to lo
ck for exclusive access: Resource temporarily unavailable)

lsof -n shows that the file is is open by imap process.