I stopped all services that were using libldap*, then started the
compiled slapd (with -u openldap anyway, and nothing else runs as that
user, though I'm not certain the libraries aren't opened before it
switches priviliges), and still I had the same symptom. (And my
compiled slapd is also believed to not use gnutls; it doesn't use
libldap anyway.)
I therefore don't see any evidence that the problem is really
gnutls-related. That you can't reproduce it doesn't quite help,
you've compiled almost everything yourself.
You think I should also try to gradually compile things myself, trying
to locate which library causes the problem? Needless to say I'd
prefer a simpler investigation path, if one exists.