Solved!.
The boot.img at \knoppix folder (a floppy syslinux boot image) must be created with the option -s (safe) to avoid this problem

A pararagraph from syslinux.doc

SYSLINUX can be used to create bootdisk images for El
Torito-compatible bootable CD-ROMs. However, it appears that many
BIOSes are very buggy when it comes to booting CD-ROMs. Some users
have reported that the following steps are helpful in making a CD-ROM
that is bootable on the largest possible number of machines:

a) Use the -s (safe, slow and stupid) option to SYSLINUX;
b) Put the boot image as close to the beginning of the
ISO 9660 filesystem as possible.

A CD-ROM is so much faster than a floppy that the -s option shouldn't
matter from a speed perspective.