I've just checked the quarantine parameter with no luck again. It only let me use the parameter once in slapd.conf file and not for every target/uri. Otherwise slaptest command gives me an error telling me that quarantine parameter is already defined.

Using it only once, slaptest command returns successfully, but the service doesn't start. I can see these errors in the log file:

May 13 10:40:29 proxyldap-pre slapd[26166]: olcDbQuarantine: value #0: quarantine already defined.

May 13 10:40:29 proxyldap-pre slapd[26166]: config error processing olcMetaSub={0}uri,olcDatabase={1}meta,cn=config: quarantine already defined

May 13 10:42:02 proxyldap-pre slapd[26194]: olcDbQuarantine: value #0: quarantine already defined.

May 13 10:42:02 proxyldap-pre slapd[26194]: config error processing olcMetaSub={0}uri,olcDatabase={1}meta,cn=config: quarantine already defined

So finally, I have to delete the parameter in the slapd.conf or I can't start the service.

Thanks again for your help. Any new ideas? It's an extrange behaviour and I don't really think OpenLDAP can't detect and manage this situation in any way.