The world is white.

KDE on my Debian testing has suddenly lost the ability to activate desktop effects. On systemsettings the message displayed was the following:

compositing is not supported on your system

I’m not a big fan of special effects like exploding windows, but plasma themes are often ugly without effects and I like to have shadows surrounding windows.
So, I have found that this is a known bug and can be solved by simply editing .kde/share/config/kwinrc. You will find this line:
CheckIsSafe=false
which you should change into:
CheckIsSafe=true
This will make you able to enable desktop effects in systemsettings again.

This experience allowed me to switch back to fluxbox again, which has been a most pleasant diversion.

For the sake of completeness, I have the following video card:
VGA compatible controller: ATI Technologies Inc RV710 [Radeon HD 4350]
and I am using proprietary drivers, but I have read about people having the same problems with other gpus too.