Grub upgrade error (Ubuntu 18.04.05)

I have initially been trying to resolve this on the forums (https://ubuntuforums.org/showthread.php?t=2461875) but have now been advised to report here as a bug

I have two Ubuntu 18.04 servers in a remote location, a month or so ago after running the normal "apt update" and "apt upgrade" commands I was faced with the error shown in the attached document.

After further reading I ran the command sudo grub-install --efi-directory=/boot/efi --target=x86_64-efi --no-nvram and it says "Installation Finished. No error reported." however anytime I run apt for anything it tries again to install grub-efi-amd64 / grub-efi-amd64-signed and results in the same error.

Here is output from the df command

Filesystem Size Used Avail Use% Mounted on
udev 16G 0 16G 0% /dev
tmpfs 3.2G 952K 3.2G 1% /run
/dev/md4 29T 29T 117G 100% /
tmpfs 16G 16K 16G 1% /dev/shm
tmpfs 5.0M 0 5.0M 0% /run/lock
tmpfs 16G 0 16G 0% /sys/fs/cgroup
/dev/md2 477M 169M 279M 38% /boot
/dev/md1 300M 3.3M 297M 2% /boot/efi
tmpfs 3.2G 0 3.2G 0% /run/user/1000

I have not risked rebooting the machines so I do not know if the above command is actually leaving the machine in a bootable state or not.