GRUB - grub_register_command_lockdown not found

Recently the OS disk of one of my servers failed. Since the OS disk is a USB stick and I have had good experiences with Diet Pi for my raspberry pi’s, I thought I’d take a look if it is also available for AMD64. Guess what, it is! So I fired up dd to push the downloaded image to the USB stick, popped the USB stick into the server and ran the OS for the first time.