Some systems were running a patch that originated from CentOS that borks their LDAP. The 3 steps to validate are:

1) Move the /etc/openldap/slapd.d folder out of the way. (The CentOS RPM creates this folder)
mv /etc/openldap/slapd.d /tmp/

2) Make sure that you have a valid slapd.conf file in the /etc/openldap/ directory. This file should have (at least) all of the schema:

# Schemas
#----------------------------------------------------------

# Core schemas
include /etc/openldap/schema/core.schema
include /etc/openldap/schema/cosine.schema
include /etc/openldap/schema/inetorgperson.schema

# ClearFoundation base
include /etc/openldap/schema/rfc2307bis.schema
include /etc/openldap/schema/clearfoundation.schema

# ClearCenter extension
include /etc/openldap/schema/clearcenter.schema

# Password policy extension
include /etc/openldap/schema/ppolicy.schema

# RADIUS extension
include /etc/openldap/schema/RADIUS-LDAPv3.schema

# Kolab extension
include /etc/openldap/schema/rfc2739.schema
include /etc/openldap/schema/kolab2.schema

# Horde extension
include /etc/openldap/schema/horde.schema

# Samba extension
include /etc/openldap/schema/samba3.schema

# OwnCloud
include /etc/openldap/schema/owncloud.schema

# Zarafa extension
include /etc/openldap/schema/zarafa.schema

# Kopano extension
include /etc/openldap/schema/kopano.schema

The Kopano is new so if the file you have doesn't have all of these and the Kopano as well, please find a valid copy in backup or as one of the files in this directory. Restore this file to its proper place.

3) Validate that the slapd.conf file is owned by ldap. If not, run:
chown ldap:ldap /etc/openldap/slapd.conf

NOTE: You can see a template copy of slapd.conf in this post: https://sfj48-fkj200.heiksthsd.cf/clearfoundation/social/community/account-manager-won-t-start,-slapd,-smb-fail-to-start-on-reboot