Lightdm fails to show login screen

[Gabriele Tozzi]

Hello, there!

When I start the PC, it boots correctly.
Lightdm starts and shows the login prompt for about 1-2 seconds, then the screen goes black, like lightdm just crashed.
If i CTRL+ALT+F5-F9, lightdm is gone, there is no more graphical screen prompting for login.
If i CTRL+ALT+F1, i can login into the terminal and inspect the problem:
- "service lightdm status" still reports lightdm as "active (running)", even if no lightdm screen can be found
- "ps aux | grep lightm" shows many lightdm-related processes still running

The log (attached) complains about being unable to autologin, but i have automatic login disabled.
I suspect this is somewhat related to suspension/resume, because it started to happen after I've tried to use it.

When I issue a "service lightdm restart", lightdm restarts and this time it works correctly (it shows the login screen, then "blinks" the screen to black after 1-2 secs like it is about to get stuck again, but this time it recovers back to the login prompt and allows me to login).

Here is my lightdm.conf:
"
[SeatDefaults]
allow-guest=false
autologin-guest=false
#autologin-user=
#greeter-session=lightdm-kde-greeter
greeter-session=lightdm-gtk-greeter
#user-session=kde-plasma
user-session=xfce4-session

[VNCServer]
enabled=false

[XDMCPServer]
enabled=false
"

Thank you for reading.

ProblemType: Bug
DistroRelease: Ubuntu 15.10
Package: lightdm 1.16.7-0ubuntu1
ProcVersionSignature: Ubuntu 4.2.0-27.32-generic 4.2.8-ckt1
Uname: Linux 4.2.0-27-generic x86_64
ApportVersion: 2.19.1-0ubuntu5
Architecture: amd64
Date: Sun Apr 3 00:10:26 2016
ProcEnviron:
 TERM=linux
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=it_IT.UTF-8
 SHELL=/bin/bash
SourcePackage: lightdm
UpgradeStatus: Upgraded to wily on 2015-10-31 (154 days ago)