I recently reinstalled a lubuntu from scratch, to upgrade to 14.04. Everything's working as expected, except for the update-manager that silently fails when trying to upgrade my system's packages.

It does correctly list the packages to be upgraded, I can then click on the "Install now" button (or something like this, I'm running a French flavour of Ubuntu). When I click the button, it seems unresponsive for 5 to 10 seconds, then the upgrade dialog with the progress bar appears, displays "Loading package list", then very shortly something about "authentication" (not long enough so that I can read it).. and closes.

The install is fresh and I have no idea where to look. I tried launching update-manager in a term but it doesn't display any error when it fails, only this message when the window first shows up :

** (update-manager:3642): WARNING **: Couldn't connect to accessibility bus: Failed to connect to socket /tmp/dbus-m0jkFlZ3yo: Connexion refusée. 

Any idea where this might come from ? I can upgrade the packages using the command line and sudo apt-get but this is not my computer, and I do not wish its owner to have to do this to upgrade the packages.

Thanks for your help !