Comment 18 for bug 1848797

This does not appear to be a new issue; but it is newly visible due to the changes in 2.04: we've now upgraded to a new major release of GRUB, and the modules are now no longer compatible with previous releases.

In other words, if you're running into this issue, your system was most likely already broken, possibly for a long while, even if it was not immediately obvious -- somehow the files on disk under /boot are no longer in sync with the copy of the bootloader installed to the MBR, or in the ESP if you're in UEFI mode.

Even though I am not sure what the actual cause of this, one possible explanation of this is modification of the system at some point in the past. For example, if /boot isn't mounted, listed in /etc/fstab; or /boot/efi in the case of UEFI. It is possible this is a side-effect of use of boot-repair tools. It's hard to know, but for now the precise cause is unknown and we will do more testing to figure out how this can happen.

In any case, right now the best thing to do to repair is to boot from a Live image (USB or CD-ROM), and follow the steps in https://help.ubuntu.com/community/Grub2/Installing#via_ChRoot to repair GRUB by hand, or reinstall.

There is additional information about this in the Debian bug report as well, here: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=931896