Comment 29 for bug 976638

Marcus Naeher has the solution in #17 above

For LXDE it is true as well that the PolicyKit Autostart has been unchecked by default in the latest version of Ubuntu/Lubuntu If you check this box and restart, Synaptic package manager will start from the launchpad menu and will ask for Authentication and all is well. Here is his answer quoted below:

"I have found the solution by happenstance:

(I am using XFCE in German, so maybe the wording I describe here is not 100% accurate)
In the XFCE settings menu -> Session management -> Automatically started applications, there is an Item "Policykit Authentication Agent".
This was unchecked. Re-check, reboot (probably logout and login is sufficient), and it is back working.

But: I am absolutely sure I did not remove the check. On one of the affected computers, I have never been in this dialog before."