This won't help very much, but I have exact same problem, openSUSE 10.3.
2 user accounts on PC. One uses kickoff, other traditional k menu, both of them click on K and screen goes grey
tried removing and adding K button to task bar, and to desktop, same problem
At least you're not alone.
i think problem started Monday after updates, there were none on Tuesday, and Wednesday night's didn't fix it.
btw at least I have learned the ctrl f1-f4 shortcuts to change workspace, and alt-f2 for the run command.
btw you can run plasma, but it just crashes again when you click on the k