@carmar: unsure what you mean, exactly. When I did try the GDM3 trick and saw the message I rebooted anyway, an eternal optimist!
I also recall when I FIRST installed lightdm i was sleepy and did not read properly, missed the option to change from gm3 to lightdm
The result was nothing.
So, do you want to me to:
- purge lightdm , then reinstall but select gdm3?
- purge lightdm and reinstall gdm3 and reboot after the error and report back?
- Something else-
- none of the above
Thks for the link, I have read it, a lot of options.
I guess yuo were referring to the green ticked re error message, if so, basically that is what I did; ignored it and rebooted. Nothing changed, went straight to clean ccccc files ccccc blocks.
The only way I seem to be able to get a graphic display is with lightdm
Hope this helps. 

