Comment 3 for bug 294389

Tried to re-installed system-tools-backends, but this did not fix the problem.
I then un-installed system-tools-backends in Synaptic. This required also un-installing the following packages.
ubuntu-desktop
fast-user-switch-applet
gnome-system-tools
gnome-applets
liboobs-1-4

I then immediately re-installed the following in Synaptic.
fast-user-switch-applet
gnome-system-tools
gnome-applets
liboobs-1-4
system-tools-backends

I did not reinstall ubuntu-desktop because it is only a meta-package that came up with a lot of dependencies I don't need or want like ekiga.
This fixed the problem with system-tools-backends. Synaptic and Update Manager run without errors and the system reboots successfully.
Hope this info helps.