Hi Soichi,
I was curious if you still encounter this issue using current OpenLDAP and back-mdb. I would note the back-bdb/hdb backends are now deprecated. It also occurred to me that the issue may have been related to lack of lock resources, etc (which are generally set in DB_CONFIG).
Regards, Quanah
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Quanah Gibson-Mount Product Architect Symas Corporation Packaged, certified, and supported LDAP solutions powered by OpenLDAP: http://www.symas.com