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.