That comes from the /etc/init/mountall-shell.conf file:
echo "Filesystem check or mount failed."
echo "A maintenance shell will now be started."
echo "CONTROL-D will terminate this shell and continue booting after re-trying"
echo "filesystems. Any further errors will be ignored"
;;
...
/sbin/sulogin
So it's likely sulogin or one of the processes it execs/forks. Try temporarily mounting the root filesystem on a working VM (or mount it after booting from the live CD), and replace that line with
/usr/bin/strace -f -o /root/sulogin-trace.log /bin/sulogin
and see what's in the log.