Hi, I used the below ldif file to add a user defined objectclass in my openldap 2.4.40 on CentOS 7.
Delete support for cn=config will be a part of the OpenLDAP 2.5 release series, as documented. You will need to slapcat your config DB, remove the offending entries, and slapadd it back.
--Quanah -- Quanah Gibson-Mount Product Architect Symas Corporation Packaged, certified, and supported LDAP solutions powered by OpenLDAP: <http://www.symas.com>