Connecting to Lubuntu desktop via VNC

I have installed Lubuntu desktop on a Ubuntu 20.03 LTS server. The desktop works fine when I log on to it locally. I can see the SDDM and I can chose between various sessions for the desktop (Lubuntu, LXDE, LXQt). However, I set up a vnc server and I configured (as per the instructions I found online) the ~/.vnc/xstartup fileto looks like this:

#!/bin/sh
xsetroot -solid grey
export XKL_XMODMAP_DISABLE=1
openbox &
/usr/bin/lxsession -s Lubuntu &

When I connect though, all I can see is a grey screen. Any ideas? Thanks a lot!