veyon-server doesn' start and no connection on Port 11100

[white_rabbit]

Hi.
We use veyon in a computer room. So far it has worked flawlessly with Ubuntu 18.04.
Now we changed to Ubuntu 20.04 and since then the teacher can't see the clients any more. Maybe it's a problem with wayland (new default?)

So far I tried:
Teacher-Computer:

systemctl status veyon.service
● veyon.service - Veyon Service
     Loaded: loaded (/lib/systemd/system/veyon.service; enabled; vendor preset: enabled)
     Active: active (running) since Tue 2025-01-21 08:51:42 CET; 35min ago

telnet localhost 11100
Trying 127.0.0.1...
Connected to localhost.
Escape character is '^]'.
RFB 003.008

veyon-server
QStandardPaths: XDG_RUNTIME_DIR not set, defaulting to '/tmp/runtime-root'
2025-01-21T09:48:57.645: [ERR] FeatureWorkerManager: can't listen on localhost!
2025-01-21T09:48:57.728: [ERR] main: Failed to start server
KLDAP: connection closed!

ps aux |grep veyon
/usr/bin/veyon-service
/usr/bin/veyon-server
 /usr/bin/veyon-worker {...}  < everything seems to be OK?!

But on a Students-Computer (Cloned -- so it's an identic installation):

systemctl status veyon.service
● veyon.service - Veyon Service
     Loaded: loaded (/lib/systemd/system/veyon.service; enabled; vendor preset: enabled)
     Active: active (running) since Tue 2025-01-21 09:20:16 CET; 23min ago

telnet localhost 11100
Trying 127.0.0.1...
telnet: Unable to connect to remote host: Connection refused

veyon-server status
qt.qpa.xcb: could not connect to display 
qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found.
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.
Available platform plugins are: eglfs, linuxfb, minimal, minimalegl, offscreen, vnc, xcb.

ps aux |grep veyon
root      387007  0.0  0.2 326184 33144 ?        SLsl 09:20   0:00 /usr/bin/veyon-service
root      387013  0.0  0.2 481748 34432 ?        Sl   09:20   0:00 /usr/bin/veyon-server

I think that the veyon-server doesn't start correctly on the Students-Clients. But why?
At the moment nobody is logged it. It was just a test via ssh / bash

Any ideas?
Thank you!

P.S.: dpkg -l |grep veyon
ii veyon 4.9.1-2~jammy

[white_rabbit]

Today I tried to run (as root):

veyon-server start

The error message looks like this:

veyon-server start
QStandardPaths: XDG_RUNTIME_DIR not set, defaulting to '/tmp/runtime-root'
22/01/2025 12:12:34 passing arg to libvncserver: -rfbport
22/01/2025 12:12:34 passing arg to libvncserver: 11200
22/01/2025 12:12:34 passing arg to libvncserver: -rfbportv6
22/01/2025 12:12:34 passing arg to libvncserver: 11200
22/01/2025 12:12:34 passing arg to libvncserver: -passwd
22/01/2025 12:12:34 x11vnc version: 4.9.0 lastmod: 2019-01-05  pid: 10067
22/01/2025 12:12:34 Using X display :0
22/01/2025 12:12:34 rootwin: 0x496 reswin: 0xe00001 dpy: 0x8c0089b0
22/01/2025 12:12:34
22/01/2025 12:12:34 ------------------ USEFUL INFORMATION ------------------
22/01/2025 12:12:34 X DAMAGE available on display, using it for polling hints.
22/01/2025 12:12:34   To disable this behavior use: '-noxdamage'
22/01/2025 12:12:34
22/01/2025 12:12:34   Most compositing window managers like 'compiz' or 'beryl'
22/01/2025 12:12:34   cause X DAMAGE to fail, and so you may not see any screen
22/01/2025 12:12:34   updates via VNC.  Either disable 'compiz' (recommended) or
22/01/2025 12:12:34   supply the x11vnc '-noxdamage' command line option.
22/01/2025 12:12:34 X COMPOSITE available on display, using it for window polling.
22/01/2025 12:12:34   To disable this behavior use: '-noxcomposite'
22/01/2025 12:12:34
22/01/2025 12:12:34 Wireframing: -wireframe mode is in effect for window moves.
22/01/2025 12:12:34   If this yields undesired behavior (poor response, painting
22/01/2025 12:12:34   errors, etc) it may be disabled:
22/01/2025 12:12:34    - use '-nowf' to disable wireframing completely.
22/01/2025 12:12:34    - use '-nowcr' to disable the Copy Rectangle after the
22/01/2025 12:12:34      moved window is released in the new position.
22/01/2025 12:12:34   Also see the -help entry for tuning parameters.
22/01/2025 12:12:34   You can press 3 Alt_L's (Left "Alt" key) in a row to
22/01/2025 12:12:34   repaint the screen, also see the -fixscreen option for
22/01/2025 12:12:34   periodic repaints.
22/01/2025 12:12:34
22/01/2025 12:12:34 XFIXES available on display, resetting cursor mode
22/01/2025 12:12:34   to: '-cursor most'.
22/01/2025 12:12:34   to disable this behavior use: '-cursor arrow'
22/01/2025 12:12:34   or '-noxfixes'.
22/01/2025 12:12:34 using XFIXES for cursor drawing.
22/01/2025 12:12:34 GrabServer control via XTEST.
22/01/2025 12:12:34
22/01/2025 12:12:34 Scroll Detection: -scrollcopyrect mode is in effect to
22/01/2025 12:12:34   use RECORD extension to try to detect scrolling windows
22/01/2025 12:12:34   (induced by either user keystroke or mouse input).
22/01/2025 12:12:34   If this yields undesired behavior (poor response, painting
22/01/2025 12:12:34   errors, etc) it may be disabled via: '-noscr'
22/01/2025 12:12:34   Also see the -help entry for tuning parameters.
22/01/2025 12:12:34   You can press 3 Alt_L's (Left "Alt" key) in a row to
22/01/2025 12:12:34   repaint the screen, also see the -fixscreen option for
22/01/2025 12:12:34   periodic repaints.
22/01/2025 12:12:34
22/01/2025 12:12:34 XKEYBOARD:
22/01/2025 12:12:34 Switching to -xkb mode to recover these keysyms:
22/01/2025 12:12:34    xkb  noxkb   Keysym  ("X" means present)
22/01/2025 12:12:34    ---  -----   -----------------------------
22/01/2025 12:12:34     X           0x40  at
22/01/2025 12:12:34     X           0x5b  bracketleft
22/01/2025 12:12:34     X           0x5d  bracketright
22/01/2025 12:12:34     X           0x7b  braceleft
22/01/2025 12:12:34     X           0x7d  braceright
22/01/2025 12:12:34     X           0x7c  bar
22/01/2025 12:12:34     X           0x5c  backslash
22/01/2025 12:12:34
22/01/2025 12:12:34   If this makes the key mapping worse you can
22/01/2025 12:12:34   disable it with the "-noxkb" option.
22/01/2025 12:12:34
22/01/2025 12:12:34
22/01/2025 12:12:34 X FBPM extension not supported.
Xlib:  extension "DPMS" missing on display ":0".
22/01/2025 12:12:34 X display is not capable of DPMS.
22/01/2025 12:12:34 --------------------------------------------------------
22/01/2025 12:12:34
22/01/2025 12:12:34 Default visual ID: 0x3b
22/01/2025 12:12:34 Read initial data from X display into framebuffer.
22/01/2025 12:12:34 initialize_xdisplay_fb: \*\*\* fb creation failed: 0x0 try: 1
22/01/2025 12:12:35 Default visual ID: 0x3b
22/01/2025 12:12:35 Read initial data from X display into framebuffer.
22/01/2025 12:12:35 initialize_xdisplay_fb: \*\*\* fb creation failed: 0x0 try: 2
22/01/2025 12:12:35 Default visual ID: 0x3b
22/01/2025 12:12:35 Read initial data from X display into framebuffer.
22/01/2025 12:12:35 initialize_xdisplay_fb: \*\*\* fb creation failed: 0x0 try: 3
22/01/2025 12:12:35 Default visual ID: 0x3b
22/01/2025 12:12:35 Read initial data from X display into framebuffer.
22/01/2025 12:12:35 initialize_xdisplay_fb: \*\*\* fb creation failed: 0x0 try: 4
22/01/2025 12:12:35 Default visual ID: 0x3b
22/01/2025 12:12:35 Read initial data from X display into framebuffer.
22/01/2025 12:12:35 initialize_xdisplay_fb: \*\*\* fb creation failed: 0x0 try: 5
2025-01-22T12:12:35.793: \[ERR\] LinuxPlatformPlugin: Received signal 11
2025-01-22T12:12:35.795: \[ERR\] BACKTRACE:
/lib/x86_64-linux-gnu/libc.so.6(+0x42520) \[0x7a7df4442520\]
/usr/lib/x86_64-linux-gnu/veyon/builtin-x11vnc-server.so(+0x7fc00) \[0x7a7ddc67fc00\]
/usr/lib/x86_64-linux-gnu/veyon/builtin-x11vnc-server.so(+0x40450) \[0x7a7ddc640450\]
/usr/lib/x86_64-linux-gnu/veyon/builtin-x11vnc-server.so(+0x145e5) \[0x7a7ddc6145e5\]
veyon-server(+0xa8f1) \[0x5802944ff8f1\]
/lib/x86_64-linux-gnu/libQt5Core.so.5(+0xccca1) \[0x7a7df4cccca1\]
/lib/x86_64-linux-gnu/libc.so.6(+0x94ac3) \[0x7a7df4494ac3\]
/lib/x86_64-linux-gnu/libc.so.6(+0x126850) \[0x7a7df4526850\]
2025-01-22T12:12:35.795: \[CRIT\] Aborting due to severe error
Abgebrochen