No idea yet, sorry. But i saw this message:
Dmitry wrote:
Warning: fd limit 1024 is lower than what Dovecot can use under full load (more than 4096). Either grow the limit or change login_max_processes_count and max_mail_processes settings
login_max_processes_count: 1024
login_max_connections: 1024
max_mail_processes: 1024
iRedMail comments out above 3 settings by default, and you turned them on. So you should fix them as suggested by dovecot (text marked in red).