Weird icons after mouseover - Colored pixels comeon top of the buttoms

I recently bought TBL and it works great, but I have one very annoying bug: when I go to the Start bar, everything's normal, but when I mouse-over the buttons, they get weird colored pixels on top of them. I used a skin before, but I've disabled it to see if I got the same problem without it. Does anybody know a solution for this?

Thank you very much in advance,
Stefan.

PS: See the file I've attached.