I installed Kubuntu 12.04 with KDE 4.8.5. Maximizing a window, or creating a new window, will make it black. It becomes OK if I decrease the size. The more active windows I have, the smaller the size of a new window has to be not to appear black. Please advise. Thanks.

You're running out of texture memory, causing windows to become invalid textures.
Disable the compositor or use xrender compositing (run "kcmshell kwincompositing" for a config dialog)

Please notice that KDe 4.8.5 is pretty dated.

Thanks! This is odd though: I deactivated "Use OpenGL 2 Shaders" in the Advanced tab; it solved the problem. Then it asked if I wanted to save the new configuration. I did not reply and did not save it. It's still active now. Yet, the problem has disappeared!