Comment 34 for bug 1851311

The problem doesn't come from the casper package. The problem is in the grub-efi-amd64-bin package (latest version tested:2.04-1ubuntu16). Actually, it's the file grubx64.efi inside this package (/usr/lib/grub/x86_64-efi/monolithic/grubx64.efi).

menuentry ".. Xubuntu .... test daily" {
 set isofile="/isos/focal-desktop-amd64.iso"
 search --set=root --file $isofile
 loopback loop $isofile
 linux (loop)/casper/vmlinuz iso-scan/filename=$isofile boot=casper noprompt quiet splash --
 initrd (loop)/casper/initrd
}

menuentry ".. Xubuntu Default cd Menu" {
 iso_path="/isos/focal-desktop-amd64.iso"
 export iso_path
 search --set=root --file $iso_path
 loopback loop $iso_path
 root=(loop)
 configfile /boot/grub/loopback.cfg
 loopback --delete loop
}

It doesn't work in 2.04 grub version. So I experienced something.

1- I downloaded the 2.02 grub-efi-amd64-bin package from here:
 https://packages.ubuntu.com/disco-updates/amd64/grub-efi-amd64-bin/download

2- I opened the deb package with my archive manager

3- I extracted the file: grubx64.efi
 (from /usr/lib/grub/x86_64-efi/monolithic/)

4- I launched efibootmgr in a terminal to be sure where to copy it

5- I copied it in the right folder in /boot/efi
 sudo cp grubx64.efi /boot/efi/EFI/xubuntu/.

After that, both menus worked fine (in a 2.02 grub)!