Authenticate window doesn't appear anymore

[Mike]

Lubuntu 11.10 64bit When I try to run Synaptic from menu, nothing goes on. I can authenticate and run the program only in console since recently.
Here is some information I have in .xsession-errors:
** (polkit-gnome-authentication-agent-1:1282): WARNING **: Unable to register authentication agent: GDBus.Error:org.freedesktop.PolicyKit1.Error.Failed: Cannot determine user of subject
Cannot register authentication agent: GDBus.Error:org.freedesktop.PolicyKit1.Error.Failed: Cannot determine user of subject
and also
** (pcmanfm:2278): DEBUG: launch command: <synaptic-pkexec>
** (pcmanfm:2278): DEBUG: sn_id = pcmanfm-2278-machine-synaptic-pkexec-30_TIME13191945
Error creating textual authentication agent: Error opening current controlling terminal for the process (`/dev/tty'): No such device or address
If I re-install sudo or dbus packages, and reboot the system, then everything is okay, but it helps only until I reboot the computer the second time. So it helps only during one session.

ProblemType: Bug
DistroRelease: Ubuntu 11.10
Package: pcmanfm 0.9.9-0ubuntu1
ProcVersionSignature: Ubuntu 3.0.0-14.23-generic 3.0.9
Uname: Linux 3.0.0-14-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 1.23-0ubuntu4
Architecture: amd64
Date: Fri Jan 6 21:13:27 2012
InstallationMedia: Lubuntu 11.10 "Oneiric Ocelot" - Release amd64 (20111012)
Pref_Config_System_lubuntu:
 [Preferred Applications]
 WebBrowser=firefox.desktop
 MailClient=
ProcEnviron:
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: pcmanfm
UpgradeStatus: No upgrade log present (probably fresh install)

[Mike (mikecom)]

This also affects all other programs requiring the authentication window, such as Time and Date, Update Manager settings.

[Launchpad Janitor (janitor)]

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

[Markus Naeher (kubuntu-user)]

I have the same problem in Xubuntu 12.04. I have 2 AMD64 PC's and an i386 Laptop. The problem occurs on one of the PC's and the Laptop.
Before I found this bug report, I already had opened my own one:
It's Bug #976638.

[Markus Naeher (kubuntu-user)]

I have a soluton, at least for XFCE:

(translated from the German version, so wording may be inaccurate)
Go to
XFCE Menu -> Settings -> Settings -> Startup and Sessions -> Programs started automatically
and activate "Policykit Authentication Agent".

Maybe there is something similar in LXDE.

But the question remains:
Why did this deactivate itself ?
At least on my Laptop, I am 100% sure that I never, ever opened this part of the settings.