Kubuntu 12.04 - computer won't power off

I've had a strange problem when powering off my home computer. It has never powered off. The blue screen with Kubuntu logo appeared and then it hung. I had to long press power off button to complete the shutdown. It was very uncomfortable.

Even stranger thing is that when I want to restart the computer it has restarted always without any problem.

Today I finally find out a solution:

Edit the kdmrc file:

$ sudo nano /etc/kde4/kdm/kdmrc

Change one line in section [Shutdown]. It looked like this:

[Shutdown]
BootManager=None
HaltCmd=/sbin/halt
RebootCmd=/sbin/reboot

So I changed it to:

[Shutdown]
BootManager=None
HaltCmd=/sbin/shutdown -P now
RebootCmd=/sbin/reboot

Now logout from KDE. Switch to console with Ctrl+Alt+F1 and restart the kdm:

$ sudo /etc/init.d/kdm stop

$ sudo /etc/init.d/kdm start

After login to KDE again, perform the shutdown. Now it works again!