OpenLDAP server cannot start - Error: check-config.sh: Read/write permissions for DB file are required

Issue

1. Error when trying to start slapd of Openldap server ldapserver:

Nov 28 21:37:04 ldapserver systemd: Failed to start OpenLDAP Server Daemon.
Nov 28 21:37:16 ldapserver systemd: Starting OpenLDAP Server Daemon...
Nov 28 21:37:16 ldapserver check-config.sh: Read/write permissions for DB file '/var/lib/ldap/alock' are required.
Nov 28 21:37:16 ldapserver check-config.sh: Read/write permissions for DB file '/var/lib/ldap/log.0000000001' are required.
Nov 28 21:37:16 ldapserver check-config.sh: Read/write permissions for DB file '/var/lib/ldap/id2entry.bdb' are required.
Nov 28 21:37:16 ldapserver check-config.sh: Read/write permissions for DB file '/var/lib/ldap/dn2id.bdb' are required.
Nov 28 21:37:16 ldapserver check-config.sh: Read/write permissions for DB file '/var/lib/ldap/objectClass.bdb' are required.
Nov 28 21:37:16 ldapserver check-config.sh: Read/write permissions for DB file '/var/lib/ldap/entryUUID.bdb' are required.
Nov 28 21:37:16 ldapserver check-config.sh: Read/write permissions for DB file '/var/lib/ldap/entryCSN.bdb' are required.
Nov 28 21:37:16 ldapserver check-config.sh: Read/write permissions for DB file '/var/lib/ldap/cn.bdb' are required.
Nov 28 21:37:16 ldapserver check-config.sh: Read/write permissions for DB file '/var/lib/ldap/log.0000000002' are required.
Nov 28 21:37:16 ldapserver check-config.sh: Read/write permissions for DB file '/var/lib/ldap/__db.001' are required.
Nov 28 21:37:16 ldapserver check-config.sh: Read/write permissions for DB file '/var/lib/ldap/__db.002' are required.
Nov 28 21:37:16 ldapserver check-config.sh: Read/write permissions for DB file '/var/lib/ldap/__db.003' are required.
Nov 28 21:37:16 ldapserver systemd: Failed to start OpenLDAP Server Daemon.

2. This is not file permission problem, as the files already have correct permission to allow ldap user to read and write:

# ls -lrt /var/lib/ldap
total 42288
-rw------- 1 ldap ldap      845 Jul 25  2018 DB_CONFIG
-rw------- 1 ldap ldap 10485760 Aug 24  2018 log.0000000001
-rw------- 1 ldap ldap   323584 Nov 23 01:20 cn.bdb
-rw------- 1 ldap ldap    36864 Nov 23 01:20 entryCSN.bdb
-rw------- 1 ldap ldap    40960 Nov 23 01:20 entryUUID.bdb
-rw------- 1 ldap ldap    69632 Nov 23 01:20 objectClass.bdb
-rw------- 1 ldap ldap   131072 Nov 23 01:20 dn2id.bdb
-rw------- 1 ldap ldap   917504 Nov 23 01:20 id2entry.bdb
-rw------- 1 ldap ldap 17457152 Nov 28 18:01 __db.002
-rw------- 1 ldap ldap  3761704 Nov 28 18:01 __db.003
-rw------- 1 ldap ldap  2367488 Nov 28 18:01 __db.001
-rw------- 1 ldap ldap 10485760 Nov 28 18:01 log.0000000002
-rwxr-xr-x 1 ldap ldap     4096 Nov 28 18:01 alock