The problem shows up, for example, when trying to de-activate the screensaver. The problem is made much worse by the fact that the screensaver seems to have a glitch which removes the password screen right after it is done showing the spinning ball for a few minutes, making it practically impossible to deactivate.
The solution is simple: unplug the network cable, and within a few seconds everything is back to normal. I do not know if Apple is aware of this bug.

