As an experiment, I disabled schroot at startup like so: aquitard# ls -l /etc/systemd/schroot.service lrwxrwxrwx 1 root root 9 Sep 5 08:53 /etc/systemd/schroot.service -> /dev/null
Rebooted the computer, made sure nothing mounted. Then I ran: aquitard# service schroot start [ ok ] Starting schroot (via systemctl): schroot.service. Which in turn mounts the schroots, followed by: aquitard# schroot -e --all-sessions Which worked this time. Which unfortunately does nothing to help explain anything. At least not for me anyway.
