Hello Imre Péntek
I've tried to reproduce the bug, but everythings work ok, so it can depend on your gnome-control-center .
Have you tried to uninstall and reinstall it?
if not, the procedure is not like a simple installation.
You have to logout, change to a tty, be carfull on dropping your network connection though pinging something like google, sudo apt-get remove --purge gnome-control-center, sudo apt-get install gnome-power-center, sudo reboot.
Let us know.