xfce GUI not works on raspi4 without display plugged in; no access with VNC

Hi,
i have installed kali on raspberry4, also X11vnc as server. With a display plugged on HDMI, remote GUI over VNC viewer works fine.
But my raspberry has to work without a display (headless).

In this case kali doesnt start the xfce GUI when there is no display plugged on HDMI. https://forums.kali.org/images/icons/icon8.png

And no GUI, also no remote access with VNC ... only access with ssh command line.
All my raspian images works fine without a display, VNC access every time ...

Any idea, whats to do to starting the GUI without a display plugged on?https://forums.kali.org/images/icons/icon11.png
Thank you