I guess AppArmor is blocking you. If I'm right, dmesg(1) will
audit messages that confirm it. Look at /etc/apparmor.d/usr.sbin.slapd
for the rules it follows.
You are correct.
You could work around AppArmor, either by copying or hard-linking
/usr/sbin/slapd somewhere else and running it from there,
This works perfectly. Thanks very much!