I'm running Kubuntu 12.04 with an AMD/ATI graphics card (and multiple monitors). After a system update about a month or two ago KSnapshot stopped working for me. All it does is capture the stock KDE desktop background (even though that isn't what I see on the screen). I cannot actually capture any application windows or any type of useful content. The screen capture feature of Gimp exhibits the same error.
The appearance of this error coincides with a Kubuntu system update (approx a month or so ago) that changed the lockscreen behavior. It appears to be a security related change. I don't know the details (or even where to find the details) of the update. But the first update was buggy and it was follow closely by another one. Both affected the lockscreen. The first update broke KSnapshot -- and monitor power management when the desktop is locked. The follow up fixed the monitor power management issue, but it did not fix the ability to capture a screen shot.
The end result for me appears to be a more secure lock screen but broken KSnapshot.
I have been waiting patiently, but I think now is the time to be proactive after waiting more than a month. I am trying to learn anything related to this issue. Google has not helped.