Openldap error: alock package is unstable
Solution 1:
This is often due to incorrect permissions on the actual db files, I don't know where openldap on Centos keep them (though I'd guess /var/lib/ldap or so), verify that the user slapd will run with has access.
If that isn't the issue, I'd try running with strace what (if any) error occurs.
Another approach is to use db_verify/db_recover to verify and salvage the db if necessary.