I have the same problem with unetbootin 608 (Arch Linux package) and syslinux 6.03pre19 when trying to boot Lubuntu 14.04 from a USB memory.
The described workaround works for me, coping the three files, libutil.c32 libcom32.c32 menu.c32 from /usr/lib/syslinux/bios/ to the USB drive root directory.
I have the same problem with unetbootin 608 (Arch Linux package) and syslinux 6.03pre19 when trying to boot Lubuntu 14.04 from a USB memory.
The described workaround works for me, coping the three files, libutil.c32 libcom32.c32 menu.c32 from /usr/lib/ syslinux/ bios/ to the USB drive root directory.