I've got the same issue on my Macbook Pro early 2008. Deleting /boot/efi/EFI/ubuntu/shimx64.efi doesn't help at all.
"Solution": Press "alt" when booting the macbook. Select "Boot from next target" and your system should boot by showing the error message. Alternatively, do exactly the same by starting a LIVE USB system at the first time.
I've got the same issue on my Macbook Pro early 2008. Deleting /boot/efi/ EFI/ubuntu/ shimx64. efi doesn't help at all.
"Solution": Press "alt" when booting the macbook. Select "Boot from next target" and your system should boot by showing the error message. Alternatively, do exactly the same by starting a LIVE USB system at the first time.