Topic: Dovecot startup problem

I'm using iRedMail 0.5.0  during the last 4 months and everything was fine.
Today I got very strange problem in dovecot.
suddenly it stop working and when I tried to restart it, it gives me the following error

Starting Dovecot Imap: /bin/bash: line 1:  5413 Alarm clock             /usr/sbin/dovecot

I goggled the eror messag and found somebody is talking about the quota plugin, If I comment the "iquota_imap and quota" in dovecot.conf everything  seem to work fine:

# IMAP configuration
protocol imap {
    #mail_plugins = quota imap_quota zlib expire
    mail_plugins = quota imap_quota zlib

    # number of connections per-user per-IP
    #mail_max_userip_connections = 10
#    mail_executable = /root/bin/TrackLastLogin.sh imap
    mail_executable = /usr/bin/TrackLastLogin.sh imap


after a while I've enabled the debug, I've see nothing wrong in the log file, so I decided to uncomment the quota plugin and check the log file in debug mood.
now the dovecot.conf is in its original case "before the problem", I have restarted the service then everything gets back to the normal case without any problem ???!!!

any idea what was the problem?

OS : CentOS 5.3
iRedmail : 0.5.0