It took some finding but the problem is the different images used.
If you use the first image then the scroll works but if you use the second image the the scroll doesn't work. This is the usal problem with pngs and unclickable links and some sizes of pngs just stop all interactions like links or scroll from working.
You can fix it by using the first image or by raising the z-index of the scroll element.
The fullproof issue is never to apply the png image to a positioned element when using the pngfix or the alpha image loader. Or simply give ie an 8bit png or gif instead.