Unable to unlock screen under lightdm in XDMCP configuration

Question

On my setup, whenever the screen gets locked, I cannot unlock it anymore. I tried all three known(to me) implementations: light-locker xfce4-screensaver xscreensaver.

Special on my setup: it's a remote desktop server using XDMCP, with the following config

#/etc/lightdm/lightdm.conf.d/xdmcp.conf
[XDMCPServer]
enabled=true
[VNCServer]
enabled=true

[LightDM]
#On a headless system, disable the automatic start of one seat so that LightDM can run in the background:
start-default-seat=false

DM is lightdm, desktop is xfce4, os is Ubuntu 20.04.6

I tried all three locker implementations because I already read other articles, like https://askubuntu.com/a/1206043/64193. in my tests, xscreensaver and xfce4-screensaver don't even blank. The screen still shows all open applications (and potentially secrets), but neither mouse nor keyboard action have any effect, and there is no way to see a password prompt.

at no time I saw a screensaver.

with light-locker, I tried to track it down further:

• with light-locker, the screen blanks when locked, but also here is no way out, no password prompt, no reaction to keyboard and mouse, except these three logs in debug mode (light-locker --debug):

[switch_greeter_timeout] gs-manager.c:430 (13:15:04):    Switch to greeter timeout
[gs_listener_send_switch_greeter] gs-listener-dbus.c:139 (13:15:04):     Send switch greeter
[listener_dbus_handle_system_message] gs-listener-dbus.c:1343 (13:15:04):        obj_path=(null) interface=(null) method=(null) destination=:1.336

• light-locker-command -l blanks the screen, but light-locker-command -a and light-locker-command -d show no reaction
• light-locker-command -q always says "The screensaver is inactive", even if the screen is blank. technically, I can't see a (moving) screensaver either, but it feels still weird

In all cases, I have to kill the active tool using killall /usr/bin/xfce4-screensaver, killall xscreensaver or killall light-locker from a console to continue working.

I did all tests with only one of them installed, so there are no interactions between them.

What I want is a simple time-based locking solution for security reasons.

The fact that all have a problems lets me think it's more a XDMCP or lightdm problem. Any idea what I can check next?