Unity session doesn't start after login from lightdm

[Shaleen Jain]

lightdm authenticates and logs in a user and a session is created but the screen hangs there forever.
Same for the guest user.

I am able to login from the other text terminals.

unity --replace results in
WARNING: no DISPLAY variable set, setting it to :0
Traceback (most recent call last):
  File "/usr/bin/unity", line 207, in <module>
    run_unity (options.verbose, options.debug, options.advanced_debug, options.compiz_path, args, options.log)
  File "/usr/bin/unity", line 129, in run_unity
    subprocess.call(["stop", "unity-panel-service"])
  File "/usr/lib/python2.7/subprocess.py", line 522, in call
    return Popen(*popenargs, **kwargs).wait()
  File "/usr/lib/python2.7/subprocess.py", line 710, in __init__
    errread, errwrite)
  File "/usr/lib/python2.7/subprocess.py", line 1335, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory

sudo unity --replace starts compiz and unity with reduced functionality i.e no applets or system bar and dbus errors.

Description: Ubuntu 15.10
Release: 15.10
lightdm:
  Installed: 1.16.6-0ubuntu1
  Candidate: 1.16.6-0ubuntu1
  Version table:
 *** 1.16.6-0ubuntu1 0
        500 http://in.archive.ubuntu.com/ubuntu/ wily-updates/main i386 Packages
        100 /var/lib/dpkg/status
     1.16.4-0ubuntu1 0
        500 http://in.archive.ubuntu.com/ubuntu/ wily/main i386 Packages

ProblemType: Bug
DistroRelease: Ubuntu 15.10
Package: lightdm 1.16.6-0ubuntu1
ProcVersionSignature: Ubuntu 4.2.0-22.27-generic 4.2.6
Uname: Linux 4.2.0-22-generic x86_64
NonfreeKernelModules: wl
ApportVersion: 2.19.1-0ubuntu5
Architecture: amd64
Date: Tue Dec 22 12:24:22 2015
EcryptfsInUse: Yes
InstallationDate: Installed on 2015-06-20 (185 days ago)
InstallationMedia: Ubuntu 14.04.1 LTS "Trusty Tahr" - Release amd64 (20140722.2)
SourcePackage: lightdm
UpgradeStatus: Upgraded to wily on 2015-12-18 (3 days ago)