OK Hugues, just seen your latest response, and updated Parallels (eSATA drive fix, which I'm booting from)
I saw the cross!!
I looked at the focussing distance, and it's 43cm, so it's the same as what I found out with the white paper and the masks. 8)
At this point there is a slight zigzag, or pixelation to the horizontal line evident, and I'd have to say there is something slightly wrong with your set up.

It is Sunday, and I'll just flip out of OS X and do a test scan now. Only flat thing I have got is an iPhone 6!