Looks like the problem comes from the chain.c32 file in raring: if I replace the file chain.c32 (/var/lib/maas/tftp/chain.c32) with the one from the syslinux package on *quantal* (/usr/lib/syslinux/chain.c32), the boot seems to work.
Looks like the problem comes from the chain.c32 file in raring: if I replace the file chain.c32 (/var/lib/ maas/tftp/ chain.c32) with the one from the syslinux package on *quantal* (/usr/lib/ syslinux/ chain.c32) , the boot seems to work.