Dear @ntsourveloudis,
Could you check if the below steps on your target and confirm if it works
On First ssh terminal
sudo service gdm3 stop
sudo service gdm stop
sudo -b X -ac -noreset
export DISPLAY=:0
sudo xrandr --fb 1920x1080 #may fail
sudo x11vnc -geometry 1920x1080 -display :0 &
Open VNC connection using VNC viewer. I would expect to see blank screen on VNC connection.
Now, open second ssh terminal, run export DISPLAY=:0 and launch any application which has GUI to see if it opens in VNC terminal.