Yes, you are right, I tried with .Xauthority, sorry. I cannot try with ~/.Xresources because I don't seem to have that file.
I tried this good idea of creating a new user. I added it to the sudo group, logged in with it through vty, run $ sudo systemctl start lightdm.service but got the dark screen too.
I am realizing now it cannot be related to users as the dark screen arrives at startup before any login.
Thank you for keeping suggesting! I am also receiving support in the issue I opened.
