Pierangelo,
back-sql fails to compile with BACKSQL_TRACE defined, I think this is the correct fix.
It also appears that objectClass searches fail (with mysql odbc) if there is no data in the ldap_entry_objclasses table. Just a simple dummy entry like this is enough to fix the problem.
+----------+----------+ | entry_id | oc_name | +----------+----------+ | 0 | dcObject | +----------+----------+
Let me know if you'd like more information.
I believe this is the issue noted here...
http://www.openldap.org/lists/openldap-software/200508/msg00450.html
Regards,
Rob