I encountered again this issue. Linux is installed in one disk, and Qubes in another disk. Both OSes were working just fine.
Until today that selecting the Qubes entry in the EFI boot menu brings up “grub>” she’ll prompt, and Qubes doesn’t boot. I deleted the Qubes entry and added it again, by issuing the command above. The entry is added, but Qubes doesn’t boot, I end up in the grub shell.