LXDM/LightDM in container: 'Seat has no VTs but VT number not 0' error

Question

I'm trying to run an x11 server in a container, using x11vnc to access it. OS container has obviously no GPU (the goal is NOT to passthrough any hardware).
I manage to launch the Display Manager (I tried with both LXDM and LightDM) and then login successfully to a lxde session.
However each time I get the following message in journalctl "pam_systemd(...): Failed to create session: Seat has no VTs but VT number not 0".
Then loginctl list-sessions is missing the new session, and also lxpolkit complains with a "No session for pid ***" error window.
My feeling is that this is related to container having no gpu hence no "real" seat but I was unable to find a solution despite multiple attempts...
I tried both LXDM and LightDM and got same problem.
LXDM conf is the standard one except the following line :
[server]
arg=/usr/bin/Xorg -noreset +extension GLX +extension RANDR +extension RENDER -config /etc/lxdm/xdummy.conf
and then LXDM LoginReady script has
/usr/bin/x11vnc -xkb -auth /var/run/lxdm/lxdm-\:0.auth -noxrecord -noxfixes -noxdamage -forever -bg -rfbport 5900 -o /var/log/x11vnc.log -display :0
My xorg conf is attached below.
Can someone point me in the right direction ?
Thanks !
xdummy.conf
Section "ServerFlags"
  Option "DontVTSwitch" "true"
  Option "AllowMouseOpenFail" "true"
  Option "PciForceNone" "true"
  Option "AutoEnableDevices" "false"
  Option "AutoAddDevices" "false"
EndSection
Section "InputDevice"
  Identifier "dummy_mouse"
  Option "CorePointer" "true"
  Driver "void"
EndSection
Section "InputDevice"
  Identifier "dummy_keyboard"
  Option "CoreKeyboard" "true"
  Driver "void"
EndSection
Section "Device"
  Identifier "dummy_videocard"
  Driver "dummy"
  Option "ConstantDPI" "true"
  VideoRam 192000
EndSection
Section "Monitor"
  Identifier "dummy_monitor"
  HorizSync   5.0 - 1000.0
  VertRefresh 5.0 - 200.0
EndSection
Section "Screen"
  Identifier "dummy_screen"
  Device "dummy_videocard"
  Monitor "dummy_monitor"
  DefaultDepth 24
  SubSection "Display"
    Viewport 0 0
    Depth 24
    Modes "1024x768" "1024x600" "800x600" "320x200"
  EndSubSection
EndSection
Section "ServerLayout"
  Identifier   "dummy_server"
  Screen       "dummy_screen"
  InputDevice  "dummy_mouse"
  InputDevice  "dummy_keyboard"
EndSection

Answer 1

Norman-
All outawater suggested is that you lop out the code that you are using
for playing sounds and make sure that works, since that seems to be
where your problem is. The rest of your (copious amounts of) applet-
based code is just a distraction at this point.
No, I have no clues as to your possible error. Though the first step that I would take, if this were my code, would be to isolate the part
that's giving me troubles and get it as simple as possible - i.e. make
sure I can write a console app that plays the midi file I want. Then I
would build up from there - i.e. dump that into an applet with one
button that does that.
Good luck to you
Lee