Comment 5 for bug 1747329

> Created USB boot stick.

This glosses over what steps you're taking here, and is the actual source of the bug.

The images that we produce are bootable images which, if copied directly to a USB stick, do not have this error.

But you are using some other tool to write the image to a USB stick, and in the process this other tool is modifying the image. It is injecting its own bootloader blocks into the disk which are incompatible with the versions that are shipped in Ubuntu 18.04.

If you are using usb-creator, than this is a duplicate of LP: #1325801 in the usb-creator package. The workaround is to not use usb-creator to write your images to the USB stick. You should use 'dd' instead, which is reliable and whose only limitation is that it doesn't set up persistent storage for your live session.

In any case this is definitely not a bug in grub, because you're not using grub when booting this image on a BIOS system; you're using isolinux which is a different bootloader.