lightdm (or greeter) only gives a blank screen instead of a login screen.

[Torfinn Ingolfsen]

Xubuntu 14.04, when the machine starts up, I don't get a login screen, just a black screen. ctrl-alt-f1 works, and if I stop lightdm
$ sudo service lightdm stop
nand start gdm
$ sudo service gdm start
I can login as normal.
Things I've tried, without success:
- reinstalling lightdm
- changing alternatives (via update-alternatives)

Before this upgrade, the machine ran Xubuntu 12.04 lts, without problems.
The machine is an Asus U35jc laptop, it has hybrid graphics (nVidia Optimus), but I am only using the open drivers (I don't use the dedicated graphics card, in Xubuntu 12.04 I just switched it off to save battery).

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: lightdm 1.10.5-0ubuntu1
ProcVersionSignature: Ubuntu 3.13.0-48.80-generic 3.13.11-ckt16
Uname: Linux 3.13.0-48-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.8
Architecture: amd64
CurrentDesktop: XFCE
Date: Mon Apr 6 23:26:20 2015
InstallationDate: Installed on 2010-10-02 (1647 days ago)
InstallationMedia: Xubuntu 10.04 "Lucid Lynx" - Release amd64 (20100429)
LightdmConfig:
 [SeatDefaults]
 greeter-session=unity-greeter
 user-session=xubuntu
LightdmGreeterLog: Failed to open /dev/null: Permission denied
LightdmGreeterLogOld: Failed to open /dev/null: Permission denied
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: lightdm
UpgradeStatus: Upgraded to trusty on 2015-04-05 (1 days ago)

[Torfinn Ingolfsen (tingox+ubuntu)]

inxi info about the machine:
tingo@kg-u35jc:~/Desktop$ date;sudo inxi -c 0 -F
Wed Apr 8 20:05:45 CEST 2015
System: Host: kg-u35jc Kernel: 3.13.0-48-generic x86_64 (64 bit) Desktop: Xfce 4.11.8 Distro: Ubuntu 14.04 trusty
Machine: System: ASUSTeK (portable) product: U35JC version: 1.0 serial: A6N0AS02239123
           Mobo: ASUSTeK model: U35JC version: 1.0 serial: BSN12345678901234567
           Bios: American Megatrends version: U35JC.208 date: 08/10/2010
CPU: Dual core Intel Core i3 CPU M 370 (-HT-MCP-) cache: 3072 KB flags: (lm nx sse sse2 sse3 sse4_1 sse4_2 ssse3 vmx)
           Clock Speeds: 1: 933.00 MHz 2: 2399.00 MHz 3: 1599.00 MHz 4: 1066.00 MHz
Graphics: Card-1: Intel Core Processor Integrated Graphics Controller
           Card-2: NVIDIA GT218M [GeForce 310M]
           X.org: 1.15.1 drivers: intel (unloaded: fbdev,vesa) FAILED: nouveau tty size: 120x40 Advanced Data: N/A for root
Audio: Card: Intel 5 Series/3400 Series High Definition Audio driver: snd_hda_intel Sound: ALSA ver: k3.13.0-48-generic
Network: Card-1: Qualcomm Atheros AR8131 Gigabit Ethernet driver: atl1c
           IF: eth0 state: down mac: 48:5b:39:8b:92:fb
           Card-2: Qualcomm Atheros AR9285 Wireless Network Adapter (PCI-Express) driver: ath9k
           IF: wlan0 state: up mac: 1c:4b:d6:fd:87:61
Drives: HDD Total Size: 640.1GB (30.7% used) 1: id: /dev/sda model: WDC_WD6400BEVT size: 640.1GB
Partition: ID: / size: 202G used: 184G (96%) fs: ext4 ID: swap-1 size: 9.34GB used: 0.00GB (0%) fs: swap
RAID: No RAID devices detected - /proc/mdstat and md_mod kernel raid module present
Sensors: System Temperatures: cpu: 67.0C mobo: N/A gpu: 69.0
           Fan Speeds (in rpm): cpu: N/A
Info: Processes: 267 Uptime: 1 day Memory: 3871.1/7773.0MB Client: Shell (sudo) inxi: 1.9.17