Unable to launch Gnome from KDM in 14.04

[My Karlsson]

Summary:

I have installed Ubuntu 14.04 on a number of machines and set them to use kdm as the display manager. This works fine except for when a user attempts to log in to a Gnome session. The greeter just disappears and nothing happens.

The user is authenticated according to auth.log:

    May 30 09:35:42 machine kdm: :0[2302]: pam_krb5(kdm:auth): user user authenticated as user@DOMAIN
    May 30 09:35:42 machine kdm: :0[2302]: pam_unix(kdm:account): unrecognized option [ignore_root]
    May 30 09:35:42 machine kdm: :0[2302]: pam_unix(kdm:session): session opened for user user by (uid=0)

But there's not a lot of Gnome processes running.

    root@machine:~# ps -fu user
    UID PID PPID C STIME TTY TIME CMD
    user 2346 2302 0 09:35 ? 00:00:00 init --user
    user 2419 2346 0 09:35 ? 00:00:00 ssh-agent -s
    user 2424 2346 0 09:35 ? 00:00:00 dbus-daemon --fork --session --address=unix:abstract=/tmp/dbus-B6YOSxHnPA
    user 2430 2346 0 09:35 ? 00:00:00 upstart-event-bridge
    user 2432 2346 0 09:35 ? 00:00:00 upstart-file-bridge --daemon --user
    user 2434 2346 0 09:35 ? 00:00:00 upstart-dbus-bridge --daemon --session --user --bus-name session
    user 2436 2346 0 09:35 ? 00:00:00 upstart-dbus-bridge --daemon --system --user --bus-name system

The Gnome session works fine when using lightdm.

1. lsb_release -rd:

Description: Ubuntu 14.04 LTS
Release: 14.04

2. apt-cache policy kdm:

kdm:
  Installed: 4:4.11.9-0ubuntu0.2
  Candidate: 4:4.11.9-0ubuntu0.2
  Version table:
 *** 4:4.11.9-0ubuntu0.2 0
        500 file:/import/ftp/ubuntu/ trusty-updates/universe amd64 Packages
        100 /var/lib/dpkg/status
     4:4.11.8-0ubuntu6 0
        500 file:/import/ftp/ubuntu/ trusty/universe amd64 Packages

3. Expected result:

User should log in to a Gnome session.

4. What actually happened:

User is stuck at a blank screen. Only the kdm background image is visible.

[Launchpad Janitor (janitor)]

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