Lubuntu 20.04 won't display the login screen after upgrading

[mahesh]

Hello,

I am new to this forum so please be patient with me.

Short story: i have a machine for remote access running Lubuntu 18.04 for almost a year. It is a headless unit that has teamviewer on it. In order to be able to connect i used "xrandr" in a script, setting a virtual display.
All worked fine until yesterday when i hit the partial upgrade button. The unit restarted and i got black screen before login screen. The OS is now Lubuntu 20.04 but i have no login screen.

All similar issues found advised me to use the dummy xorg display driver ([COLOR=var(--black-800)]xserver-xorg-video-dummy[/COLOR]). This did not help. The unit will not boot into login screen at all.
My script is now advising me that "Can't open display".

Teamviewer is starting and allows me to connect to a black 640x480 screen. I can use Ctrl+Alt+F1 to get the tty1 on screen.

After further reseach, i found the following command: "killall Xorg"
Running this command brings me the login screen and then i can login and do my stuff without any issues. But rebooting the computer, i get the same black screen. Before this "partial upgrade" all went fine and i had no need to "killall Xorg".
I am using lightdm.

Can you, please, guide me to fix this?
I am a beginner and i want to understand why this is now broken. Also, i need to understand why does this "killall Xorg" command really does and how to script it in order to be executed automatically (if this is best practice, of course).

Please feel free to ask any other information you may need.

Thank you!
Best regards,
Marian