I guess AppArmor is blocking you. If I'm right, dmesg(1) will show some 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!