I have had this issue with KDE on (i think) every distro i have used. The Kmenu has a transparent cut-out (seemingly randomly). I can manage to temperarly fix the problem by waiving the pointer over the kmenu. See pictures below:
Normal:
Error:
Note: I had to take pictures with my camera, since the problem went away with a screen capture. And the problem remains, reguardless of weather the kicker is transparent. I should also mention, that I never have this problem with the default kde icon theme.
Thanks
Fonz