Ubuntu 19.04 VNC stuck after login screen of LightDM

I'm trying to setup a box as a remote server and am stuck configuring VNC server now.

The box has no discrete GPU and is running on UHD Graphics 630 shipped with Core i7-8750H, and it's set up with Ubuntu 19.04 (Disco Dingo) and an active internet connection (no firewall so far). There was a monitor during the initial setup (OS installation etc.), but has since been removed (the box itself was moved to another place), so it's only accessible via SSH now.

I installed LightDM, X11VNC and xserver-xorg-video-dummy, and configured a dummy display, mostly following this answer.

I connect this headless box with RealVNC (on my Windows PC) and entered my password on the login screen, and got a black screen with only the wallpaper and nothing to interact with.

I'm trying to find out a way to get a functional desktop environment, something like this:

This VNC server works perfectly fine if I attach a monitor to the box and login with another physical keyboard attached together, but this is not applicable anymore after the initial setup because I intend to use it as a remote server with internet as the only external I/O device. (And this is how I grabbed the above screenshot.)

I suppose this issue would be identical with Bionic (18.04 LTS) and Cosmic (18.10).