I had exactly the same problem and solved it by reinstalling all dependencies for software-properties-gtk package: python, python-central, synaptic, gksu, python-software-properties, python-glade2, python-gtk2, update-manager and software-properties-gtk itself. Everything is well now!
I had exactly the same problem and solved it by reinstalling all dependencies for software- properties- gtk package: python, python-central, synaptic, gksu, python- software- properties, python-glade2, python-gtk2, update-manager and software- properties- gtk itself. Everything is well now!