Oh that would be bad… really bad… 
This error:
as i remember displays often when someone installed grub in efi mode first and then “to repair it”, one installed (maybe be accident) grub in BIOS mode. Grub tries to write the MBR at the beginning and that could indeed destroy the first partition @maxemilian .
I guess you have to recreate the efi partition, correct the UUID at /etc/fstab, chroot and reinstall grub in efi mode like @Aragorn has described above.