Hi there
If someone else reads this and they aren’t using autoexec.ipxe (just the default.ipxe like me) and still has this error I’ve got the solution that worked for me:

I had to downgrade the GRUB version from 2.12 to 2.06 in my Linux images (mostly Kali Linux).
Version 2.12 has a bug (bug #67078) which is mentioned here: https://savannah.gnu.org/bugs/?67078

The downgrade works as follows:
Create a pin file: sudo vim /etc/apt/preferences.d/grub-pin
Content of the file:

Package: grub* grub2*
Pin: version 2.06*
Pin-Priority: 1001

sudo apt update
Downgrade grub packages: sudo apt full-upgrade
sudo grub-install
sudo update-grub
verify the installed version: dpkg -l | grep grub-common

You can now capture and deploy the the image.

Edit: of course I am and was using the latest FOG 1.5.10.1667 with newest Kernel and Initrd versions.