Hi again.
Seems that my case is connected to my NVIDIA card and this bug:
https://bugs.launchpad.net/ubuntu/+source/xorg-server/+bug/1525735
The following solved my problem:
sudo apt install xserver-xorg-legacy
In /etc/X11/Xwrapper.config I replaced
allowed_users=console
with
allowed_users=
I am affected by the same bug, although the X error message is slightly
different ("parse_vt_settings: Cannot open /dev/tty0" instead of
"xf86OpenConsole: " ...)
In /etc/X11/Xwrapper.config I have tried to replaced
allowed_users=console
with
allowed_users=anybody
and in this file I also tried
