Our tests with hdb haven't improve the situation. Even after a few binds from our Java application or thanks to ldapsearch, OpenLDAP doesn't respond. We did those tests on RHEL 5.4 + OpenLDAP 2.4.26 + Berkeley DB 4.8.30. We also got the freeze with BDB 5.1.25, but didn't test with BDB 5.2.28 ..
So, we'll now build with debugging symbols and no optimization and follow your other recommandations ..