onboard under novnc: No X keyboard found, retrying...

Installed onboard on ubuntu 24.04.1 with xfce4.

When staring the app, the space key shows the message: "No X keyboard found, retrying..."
Most of the other keys are "?"
Pressing any of the "?" returns a key but not what you'd expect from the key position.

Any suggestion how to debug?

[edit] seems the same issue as https://bugs.launchpad.net/onboard/+bug/1001736

Starting onboard from a terminal produces this:

15:55:24.423 WARNING Config: mousetweaks GSettings schema not found, mousetweaks integration disabled.
15:55:24.498 WARNING OnboardGtk: vk: XkbQueryExtension failed: compiled for v1.0 but found v0.0
15:55:24.520 WARNING Onboard.XInput: Failed to create osk.Devices: failed to initialize XInput extension
15:55:24.520 WARNING TouchInput: XInput event source failed to initialize, falling back to GTK.
15:55:24.520 WARNING Onboard.Keyboard: XInput click simulator CSFloatingSlave unavailable, falling back to CSButtonMapper.
15:55:24.586 WARNING TouchInput: XInput event source failed to initialize, falling back to GTK.

(onboard:12626): Gdk-CRITICAL **: 15:55:24.770: gdk_window_thaw_toplevel_updates: assertion 'window->update_and_descendants_freeze_count > 0' failed
15:55:26.111 WARNING OnboardGtk: vk: XkbQueryExtension failed: compiled for v1.0 but found v0.0
15:55:27.112 WARNING OnboardGtk: vk: XkbQueryExtension failed: compiled for v1.0 but found v0.0
15:55:28.112 WARNING OnboardGtk: vk: XkbQueryExtension failed: compiled for v1.0 but found v0.0
15:55:29.112 WARNING OnboardGtk: vk: XkbQueryExtension failed: compiled for v1.0 but found v0.0