Attempting to lock screen sometimes kills whole session

[SB]

Attempting to lock screen sometimes kicks me to lightdm login screen (like I rebooted) insteading of locking the screen, that effectively kills active session because when I log in again I get an "empty" session, like I rebooted the system.

lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 14.04.2 LTS
Release: 14.04
Codename: trusty

apt-cache policy lightdm
lightdm:
  Installed: 1.10.5-0ubuntu1
  Candidate: 1.10.5-0ubuntu1
  Version table:
 *** 1.10.5-0ubuntu1 0
        500 http://archive.ubuntu.com/ubuntu/ trusty-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     1.10.0-0ubuntu3 0
        500 http://archive.ubuntu.com/ubuntu/ trusty/main amd64 Packages

apt-cache policy unity
unity:
  Installed: 7.2.4+14.04.20150316-0ubuntu1
  Candidate: 7.2.4+14.04.20150316-0ubuntu1
  Version table:
 *** 7.2.4+14.04.20150316-0ubuntu1 0
        500 http://archive.ubuntu.com/ubuntu/ trusty-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     7.2.2+14.04.20140714-0ubuntu1.1 0
        500 http://archive.ubuntu.com/ubuntu/ trusty-security/main amd64 Packages
     7.2.0+14.04.20140416-0ubuntu1 0
        500 http://archive.ubuntu.com/ubuntu/ trusty/main amd64 Packages

uname -r
3.16.0-38-generic

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: lightdm 1.10.5-0ubuntu1
ProcVersionSignature: Ubuntu 3.16.0-38.52~14.04.1-generic 3.16.7-ckt10
Uname: Linux 3.16.0-38-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.11
Architecture: amd64
CurrentDesktop: Unity
Date: Wed May 27 01:06:54 2015
InstallationDate: Installed on 2015-05-17 (9 days ago)
InstallationMedia: Ubuntu 14.04.2 LTS "Trusty Tahr" - Release amd64 (20150218.1)
SourcePackage: lightdm
UpgradeStatus: No upgrade log present (probably fresh install)

[Kai Mast (kai-mast)]

I had a similar issue. Did you have unity-system-compositor installed by any chance? Removing it solved the problem for me.

[SB (emehntehtt)]

I dont know, maybe, I cant check now as I installed 15.04 in the mean time, as I didnt find a fix for this issue. Is unity-system-compositor a dependency for some package? I had this issue with 14.04.2 but not with 14.04/14.04.1 LTS.