package lightdm 1.8.2-0ubuntu1 failed to install/upgrade: subprocess installed post-removal script returned error exit status 1

[Guendalf]

When upgrading from 13.04 (raring) to 13.10 (saucy), the graphical upgrade failed.
After looking at the error messages from apt-get, it appears that lightdm was not correctly configured.
This is the workaround I applied before rebooting:
# killall lightdm
# apt-get remove --purge lightdm
# apt-get install desktop-ubuntu (and select lightdm as the display manager)

ProblemType: Package
DistroRelease: Ubuntu 13.10
Package: lightdm 1.8.2-0ubuntu1
ProcVersionSignature: Ubuntu 3.11.0-12.19-generic 3.11.3
Uname: Linux 3.11.0-12-generic x86_64
ApportVersion: 2.12.5-0ubuntu2
AptOrdering:
 python-appindicator: Install
 lightdm: Configure
 python-appindicator: Configure
 ubuntu-desktop: Configure
Architecture: amd64
Date: Sun Oct 20 01:58:55 2013
DuplicateSignature: package:lightdm:1.8.2-0ubuntu1:le sous-processus script post-installation installé a retourné une erreur de sortie d'état 1
ErrorMessage: le sous-processus script post-installation installé a retourné une erreur de sortie d'état 1
InstallationDate: Installed on 2010-10-13 (1102 days ago)
InstallationMedia: Ubuntu 10.10 "Maverick Meerkat" - Release amd64 (20101007)
LightdmConfig:
 [SeatDefaults]
 greeter-session=unity-greeter
 user-session=ubuntu
 autologin-user=johan
MarkForUpload: True
SourcePackage: lightdm
Title: package lightdm 1.8.2-0ubuntu1 failed to install/upgrade: le sous-processus script post-installation installé a retourné une erreur de sortie d'état 1
UpgradeStatus: Upgraded to saucy on 2013-10-19 (0 days ago)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.