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In OpenLDAP config file slapd.conf, update parameter loglevel to value 256, then restart OpenLDAP service.
/etc/openldap/slapd.conf/etc/ldap/slapd.conf/usr/local/etc/openldap/slapd.confloglevel 256
OpenLDAP is configured by iRedMail to log into /var/log/openldap.log by default.