"Boot error" is a message that is embedded in syslinux, NOT in grub4dos.
Try entering the single lines on grub4dos command line and see how it responds, i.e. press "c" in grub4dos command line, then TYPE:
find --set-root /syslinux.bs[ENTER]
and post results.
You do not have by any chance ANOTHER driver with file "syslinux.bs" in ROOT, right?