Thanks for the report. I've reproduced the bug; it appears to be
slapo-rwm(5). The core dump is related to a double free, which I'm=20
trying to track.
Please test the latest commit of overlays/rwm.c (should apply fine to
2.4.23) and report. It fixes the double free.
With respect to your problem description, "lastmod off" means that the
backend will not *generate* modification-related operational attributes;
however they are proxied. Moreover, you can't strip them off with
mapping unless the remapping is part of a replication process (i.e. the
backend contains syncrepl or "updatedn" is set and the operation is
performed with that identity).