Desktop spontaneously logs out a few seconds after logging in

[mb]

Steps to reproduce:

1) Leave system idle until screen locks and monitor goes dark
2) Wiggle the mouse to wake up the monitor
3) Enter password to unlock the screen

Automatically locks the screen again within 5 to 15 seconds and monitor goes dark.

Unlocking the screen a second time works fine, and the spontaneous screen lock doesn't happen as long as I'm actively using the system.

But if I let it go idle again and the screen locks, then the first unlock attempt only lasts a few seconds before having to re-enter the password.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: unity 7.1.2+14.04.20140313-0ubuntu1
ProcVersionSignature: Ubuntu 3.13.0-17.37-generic 3.13.6
Uname: Linux 3.13.0-17-generic x86_64
ApportVersion: 2.13.3-0ubuntu1
Architecture: amd64
CompizPlugins: No value set for `/apps/compiz-1/general/screen0/options/active_plugins'
CurrentDesktop: Unity
Date: Sun Mar 16 19:04:28 2014
InstallationDate: Installed on 2014-03-08 (8 days ago)
InstallationMedia: Ubuntu 14.04 LTS "Trusty Tahr" - Alpha amd64 (20140308)
ProcEnviron:
 LANGUAGE=en_US
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: unity
UpgradeStatus: No upgrade log present (probably fresh install)

[Launchpad Janitor (janitor)]

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