Paul, I just noticed the cable is daaged! It was cratching agains the video board, as mentioned in a faq somewhere. I did check it, but when I did it was looking fine.
So, fortunately I have another one, but not here where I am. I will have to wait until the weekend to update.

Thank youf ro your help. Now, I really hope it is the cable, at least... I can see the wire underneat through the scratched surface of the cable, so there are good chances that that's where the problem is.