Comment 12 for bug 833058

Exactly. I was wrong with Synaptic, I guess it can't be used for update, but indeed I installed kubuntu-desktop on top of other DE, so I still can use updater from Ubuntu. Anyway, this should be solved by simply
sudo apt-get update & sudo apt-get upgrade
or a better solution: a new fixed version of muon-updater should be released that would install along with kubuntu-desktop.
I have kubuntu-desktop 4.8.1 from backports.