try this one maybe?
chown iredadmin:vmail /usr/lib/dovecot/managesieve
(assuming iredadmin is a member of the vmail group)
If that doesn't work, you might try o set it to whatever permissions managesieve might need.
Because of that error, it sure seems like it's something related. (if there is a log-level or something to get extra details out of the logs, you might turn that one as well)