The rootbinddn is available in the file /etc/ldap.conf that just can affect the OpenLDAP clients and not the slapd daemon (OpenLDAP server). It is strange that modifying it in Zeroshell it fixes your problem.
In any case, do not forget that Zeroshell cannot authenticate by using LDAP. The authentication is delegated to Kerberos5.
Regards
Fulvio