Sounds like something has corrupted a Grub file, meaning Grub needs to be reinstalled. It's bizarre that the same issue would affect more than one machine, though, as the problem is not very common.
Try this from Terminal:
sudo apt install --reinstall grub
sudo apt install --reinstall grub2-common
sudo apt install --reinstall grub-efi-amd64
sudo apt install --reinstall grub-efi-amd64-bin
sudo apt autoremove
sudo apt install grub-efi-amd64-signed
Once this is done, do a dist-upgrade
:
sudo apt update
sudo apt dist-upgrade
This should resolve the issue 👍🏻