I’ve solved the problem myself. It seems that the machine i was using for testing is not compatible.
I have just tested with two different models and all works well.
The incompatible model is a Lenovo A70z (1165-ABG). The config im using is:
LABEL NXTOP 41 Engine Installer
MENU NXTOP 41 Engine Installer
kernel mboot.c32
append fog/nxtop41/xen.gz — fog/nxtop41/vmlinuz boot=casper webboot=[url]http://ip-of-server/fog/nxtop41/nxtopengine.iso[/url] ro quiet console=tty8 — fog/nxtop41/initrd.img –
Maybe someone else has any use of this information.