Problem with starting containers on Ubuntu 20.04

#  lxc config show --expanded ud
architecture: x86_64
config:
  boot.autostart: "true"
  boot.autostart.delay: "60"
  image.architecture: amd64
  image.description: Debian jessie amd64 (20200504_05:24)
  image.os: debian
  image.release: jessie
  raw.lxc: |-
    lxc.net.0.ipv4.address = 10.0.100.37/20
    lxc.net.0.ipv4.gateway = 10.0.100.3
  volatile.base_image: 3cac6eacbbc5824812fc576fcf09b7f7bad95ed1f1109a41c5c7575274958bd3
  volatile.br0.host_name: vethe3546e06
  volatile.br0.hwaddr: 00:16:3e:07:59:28
  volatile.br0.name: eth1
  volatile.idmap.base: "0"
  volatile.idmap.current: '[{"Isuid":true,"Isgid":false,"Hostid":1000000,"Nsid":0,"Maprange":1000000000},{"Isuid":false,"Isgid":true,"Hostid":1000000,"Nsid":0,"Maprange":1000000000}]'
  volatile.idmap.next: '[{"Isuid":true,"Isgid":false,"Hostid":1000000,"Nsid":0,"Maprange":1000000000},{"Isuid":false,"Isgid":true,"Hostid":1000000,"Nsid":0,"Maprange":1000000000}]'
  volatile.last_state.idmap: '[{"Isuid":true,"Isgid":false,"Hostid":1000000,"Nsid":0,"Maprange":1000000000},{"Isuid":false,"Isgid":true,"Hostid":1000000,"Nsid":0,"Maprange":1000000000}]'
  volatile.last_state.power: RUNNING
devices:
  br0:
    nictype: bridged
    parent: br0
    type: nic
  root:
    path: /
    pool: default
    type: disk
ephemeral: false
profiles:
- default
stateful: false
description: ""

There being mounts under rootfs/ is very unusual and the sign of an issue…

You may want to do nsenter --mount=/run/snapd/ns/lxd.mnt then run cat /proc/self/mountinfo, locate any of those mounts where you have things mounted below rootfs and unmount those. That should clear the issue, though it doesn’t really explain where those came from in the first place…