Why I can't enable compositing option in KDE4?

[Yuri]

I have the latest NVidia driver and the card is 9400GT.

In "Configure Window Behavior" screen I first check "Enable desktop 
effects".
Then it says that "Compositing is temporarily disabled". When I click 
"Resume Compositing" sometimes nothing happens at all. But sometimes 
after few clicks it says that it's now enabled. But after that when I 
open this screen again it says it is "temporarily disabled" again. No 
explanation is given.

xorg.conf has this section:
Section "Extensions"
  Option "Composite" "Enable"
  Option "MIT-SHM" "Enable"
EndSection

And Xorg.log acknowledges this:
(**) Extension "Composite" is enabled
(**) Extension "MIT-SHM" is enabled

What's wrong?
Compositing and effects used to work almost fine before. Except for the 
black windows problem.

Yuri