I should have told that the images I used were hand-made, meaning that they were created after the instructions given in the howto.
The result is the not working swap in the domU.
However, I created an image with the "xen-create-image" command, giving me two files, "disk.img" and "swap.img".
The strange thing is that the domU using the script-made images works perfectly now, meaning that it even uses swap (what the other domU's didn't).
Does someone have any clue why the hand-made images don't work and the others do ?