Hello all,
The fix for the Issue 9264 (Add lock to slapo-unique to delay new ops until current op is complete) https://bugs.openldap.org/show_bug.cgi?id=9264 prevents an inconsistent or unexpected behavior of slapd in the case of multiple concurrent modifiers, thus I would suggest to backport the fix to the OPENLDAP_REL_ENG_2_4.
The conditions that trigger this is not that rare and we have encountered it multiple times, mostly but not only during batch operations
Best Regards, Nikos