With UCS 4.0 the UEFI components of UCS have been updated. After updating UCS 3.2 UEFI systems a number of steps have to be performed as user root to make sure the UCS system remains bootable:
A few new packages have to be installed and one outdated package has to be removed (if still installed).
univention-install shim-signed grub-efi-amd64 grub-efi-amd64-signed
Next the file /etc/fstab has to be modified. Instead of the suggested editor vim any other editor may be used.
Please modify the entry for /boot/grub. The partition has to be mounted on /boot/efi instead.
Finally, the new mountpoint has to be created and the GRUB installation updated:
find /boot/efi/ -mindepth 1 -xdev -delete