This has been tracked down to a mis-build of debian-installer, caused by a network blip leading to wget to fail to grab the correct grub-signed bits from the Ubuntu archive.
A simple rebuild of d-i and the image will correct the problem. But the d-i code should also be improved to treat a failing wget here as a build failure.
This has been tracked down to a mis-build of debian-installer, caused by a network blip leading to wget to fail to grab the correct grub-signed bits from the Ubuntu archive.
A simple rebuild of d-i and the image will correct the problem. But the d-i code should also be improved to treat a failing wget here as a build failure.