This is a GRUB problem, not a DUET problem. (DUET seems to be working correctly, based on the description.) It sounds like GRUB isn't finding its configuration file, which is odd if this is a standard Ubuntu installation with its usual version of GRUB; but if you did anything unusual to isntall GRUB, it could be you need to re-visit the GRUB installation options. Also, Ubuntu normally installs GRUB to the ESP's
EFI\ubuntu\grubx64.efi. Thus, you might try launching that file rather than
EFI\boot\bootx64.efi. The two files might actually be identical, but if they're not, using
EFI\ubuntu\grubx64.efi might work better.
Unfortunately, GRUB is very tricky to configure correctly. It works easily for many users only because most distributions include complex scripts that scan the system and create a configuration that works correctly. Instead of working on GRUB, you might consider using another EFI boot loader. My Web page on this topic covers the options. The easiest to get started with is likely to be my rEFInd boot manager in conjunction with the EFI stub loader (which is built into 3.3.0 and later kernels). If you're using the original Ubuntu 12.04, it came with a 3.2.0 kernel, so the EFI stub loader won't work; but the 12.04.2 update comes with a later kernel that does have EFI stub loader support.