I noticed this crash 4 times today. To recreate, ran the following commands (almost always in this order)

5.11.0-41-generic | 20.04.1-Ubuntu | x86_64 | x86_64 | x86_64

$ sudo lxc-ls
$ sudo lxc-start -n test
$ sudo lxc-ls -f
$ sudo lxc-console -n test
$ sudo lxc-stop test
$ sudo lxc-ls -f
$ sudo lxc-ls -f

syslog:
Dec  1 19:09:53 ThinkPad kernel: [ 4503.306174] kauditd_printk_skb: 24 callbacks suppressed
Dec  1 19:09:53 ThinkPad kernel: [ 4503.306177] audit: type=1400 audit(1638365993.337:184): apparmor="DENIED" operation="mount" info="failed flags match" error=-13 profile="lxc-container-default-cgns" name="/" pid=22258 comm="mount" flags="rw, remount"
Dec  1 19:09:53 ThinkPad kernel: [ 4503.325009] audit: type=1400 audit(1638365993.357:185): apparmor="DENIED" operation="mount" info="failed flags match" error=-13 profile="lxc-container-default-cgns" name="/snap/" pid=22277 comm="mount" flags="rw, shared"
Dec  1 19:09:53 ThinkPad kernel: [ 4503.325689] audit: type=1400 audit(1638365993.357:186): apparmor="DENIED" operation="mount" info="failed flags match" error=-13 profile="lxc-container-default-cgns" name="/" pid=22278 comm="(md-udevd)" flags="rw, rslave"
Dec  1 19:09:56 ThinkPad kernel: [ 4506.767508] audit: type=1400 audit(1638365996.801:187): apparmor="DENIED" operation="mount" info="failed flags match" error=-13 profile="lxc-container-default-cgns" name="/" pid=22302 comm="(md-udevd)" flags="rw, rslave"
Dec  1 19:10:40 ThinkPad kernel: [ 4550.606075] lxcbr0: port 1(vethEY9smt) entered disabled state
Dec  1 19:10:40 ThinkPad kernel: [ 4550.607657] device vethEY9smt left promiscuous mode
Dec  1 19:10:40 ThinkPad kernel: [ 4550.607661] lxcbr0: port 1(vethEY9smt) entered disabled state
Dec  1 19:11:09 ThinkPad kernel: [ 4579.319750] systemd[22860]: NetworkManager-dispatcher.service: Failed to connect stdout to the journal socket, ignoring: Connection refused
Dec  1 19:12:09 ThinkPad kernel: [ 4639.237516] fbcon: Taking over console
Dec  1 19:12:09 ThinkPad kernel: [ 4639.243508] Console: switching to colour frame buffer device 240x67
Dec  1 19:12:09 ThinkPad kernel: [ 4639.522393] systemd[23043]: user-runtime-dir@125.service: Failed to connect stdout to the journal socket, ignoring: Connection refused
Dec  1 19:12:09 ThinkPad kernel: [ 4639.558561] systemd[23062]: user@125.service: Failed to connect stdout to the journal socket, ignoring: Connection refused


apport.log:
ERROR: apport (pid 22986) Wed Dec  1 19:12:08 2021: called for pid 916, signal 6, core limit 0, dump mode 1
ERROR: apport (pid 22986) Wed Dec  1 19:12:08 2021: executable: /usr/lib/systemd/systemd-timesyncd (command line "/lib/systemd/systemd-timesyncd")
ERROR: apport (pid 22986) Wed Dec  1 19:12:08 2021: is_closing_session(): no DBUS_SESSION_BUS_ADDRESS in environment
ERROR: apport (pid 22986) Wed Dec  1 19:12:08 2021: apport: report /var/crash/_usr_lib_systemd_systemd-timesyncd.102.crash already exists and unseen, doing nothing to avoid disk usage DoS
ERROR: apport (pid 22992) Wed Dec  1 19:12:09 2021: called for pid 1007, signal 6, core limit 0, dump mode 1
ERROR: apport (pid 22992) Wed Dec  1 19:12:09 2021: executable: /usr/lib/systemd/systemd-logind (command line "/lib/systemd/systemd-logind")
ERROR: apport (pid 22992) Wed Dec  1 19:12:09 2021: is_closing_session(): no DBUS_SESSION_BUS_ADDRESS in environment
ERROR: apport (pid 22992) Wed Dec  1 19:12:09 2021: apport: report /var/crash/_usr_lib_systemd_systemd-logind.0.crash already exists and unseen, doing nothing to avoid disk usage DoS
ERROR: apport (pid 23289) Wed Dec  1 19:12:42 2021: called for pid 915, signal 6, core limit 0, dump mode 1
ERROR: apport (pid 23289) Wed Dec  1 19:12:42 2021: executable: /usr/lib/systemd/systemd-resolved (command line "/lib/systemd/systemd-resolved")
ERROR: apport (pid 23289) Wed Dec  1 19:12:42 2021: is_closing_session(): no DBUS_SESSION_BUS_ADDRESS in environment
ERROR: apport (pid 23289) Wed Dec  1 19:12:42 2021: apport: report /var/crash/_usr_lib_systemd_systemd-resolved.101.crash already exists and unseen, doing nothing to avoid disk usage DoS
ERROR: apport (pid 23299) Wed Dec  1 19:12:53 2021: called for pid 658, signal 6, core limit 0, dump mode 1
ERROR: apport (pid 23299) Wed Dec  1 19:12:53 2021: executable: /usr/lib/systemd/systemd-udevd (command line "/lib/systemd/systemd-udevd")
ERROR: apport (pid 23299) Wed Dec  1 19:12:53 2021: is_closing_session(): no DBUS_SESSION_BUS_ADDRESS in environment
ERROR: apport (pid 23299) Wed Dec  1 19:12:53 2021: apport: report /var/crash/_usr_lib_systemd_systemd-udevd.0.crash already exists and unseen, doing nothing to avoid disk usage DoS