The bisect will almost guaranteed show a change at the change to the new
setup code. The message means that the setup code wasn't loaded
correctly into memory; the big question is *why*.
What distro/version of grub are you running? I'm wondering if there are
some old version of grub out there which did the "load four sectors"
way-anciently-obsolete crap; the other possibility that comes to mind is
setting up the stack in an invalid manner.