While using the "cn=config" configuration, I found that "cn=config" is also the default olcRootDN. By just configuring a olcRootPW , login to the config database is possible using "cn=config" and the configured olcRootPW. This default should be documented in the admin guide, etc.
Fixed in HEAD's slapd-config(5); may be worth mentioning in the admin guide as well.
Thanks, p.