Software Updater (update-manager) window goes away, updates are not installed

[FernanAguero]

This problem started some weeks or months ago, I can never install updates running the 'Software Updater' (update-manager). The updater launches OK, it checks for updates correctly, but upon clicking the 'Install Now' button, a brief message reads "waiting for authentication" and then the window goes away, and no updates are installed.

Running the update-manager from a terminal does not produce any errors or warnings. Everything happens as explained above, and the program silently fails to install updates.

However, running apt-get update + apt-get upgrade works perfectly from the terminal.

The problem is similar to the one reported here,
https://askubuntu.com/questions/546363/software-updater-software-updates-not-working-on-14-04-lts

but the solution suggested (installing a threading.py file) does not work for me. I have already a threading.py file in my /usr/lib/python3.4 directory, but replacing it with a newer version from https://hg.python.org/cpython/file/3.4/Lib/threading.py did not fix the issue.

Please help me debug further or point me to a solution.

lsb_release -rd
Description: Ubuntu 14.04.4 LTS
Release: 14.04

apt-cache policy update-manager
update-manager:
  Installed: 1:0.196.14
  Candidate: 1:0.196.14
  Version table:
 *** 1:0.196.14 0
        500 http://archive.ubuntu.com/ubuntu/ trusty-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     1:0.196.11 0
        500 http://archive.ubuntu.com/ubuntu/ trusty/main amd64 Packages

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: update-manager 1:0.196.14
ProcVersionSignature: Ubuntu 3.13.0-83.127-generic 3.13.11-ckt35
Uname: Linux 3.13.0-83-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.19
Architecture: amd64
CurrentDesktop: Unity
Date: Thu Apr 21 09:07:37 2016
InstallationDate: Installed on 2012-10-29 (1269 days ago)
InstallationMedia: Ubuntu 12.04.1 LTS "Precise Pangolin" - Release amd64 (20120823.1)
PackageArchitecture: all
SourcePackage: update-manager
UpgradeStatus: Upgraded to trusty on 2014-11-25 (512 days ago)

[Launchpad Janitor (janitor)]

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