GUI Apps in LXD container

It looks to me that it is trying to send to the abstract Unix socket of X11.
I do not think I have published yet a tutorial on using the abstract socket. Which guide are you using?

You can try the following LXD profile. Here you do not need to manually install the client libraries for Nvidia; they are automatically provided by LXD by downloading the NVidia runtime. Also, we use a proxy device to share the X11 Unix socket of the host to the container.

$ lxc profile show guinvidia
config:
  environment.DISPLAY: :0
  nvidia.driver.capabilities: graphics, compute, display, utility, video
  nvidia.runtime: "true"
  raw.idmap: both 1000 1000
  user.user-data: |
    #cloud-config
    runcmd:
      - 'sed -i "s/; enable-shm = yes/enable-shm = no/g" /etc/pulse/client.conf'
      - 'echo export PULSE_SERVER=unix:/tmp/.pulse-native | tee --append /home/ubuntu/.profile'
    packages:
      - x11-apps
      - x11-utils
      - mesa-utils
      - pulseaudio
description: GUI LXD profile
devices:
  PASocket:
    path: /tmp/.pulse-native
    source: /run/user/1000/pulse/native
    type: disk
  X0:
    bind: container
    connect: unix:/tmp/.X11-unix/X1
    gid: "1000"
    listen: unix:/tmp/.X11-unix/X0
    mode: "0777"
    security.gid: "1000"
    security.uid: "1000"
    type: proxy
    uid: "1000"
  mygpu:
    type: gpu
name: guinvidia
used_by: