[CentOS] update to Centos7.4: Failed to open \EFI\BOOT\grubx64.efi - Not Found

Hello All, 

updating from Centos7.3 to Centos7.4 rendered one of our laptops unbootable. 
EM: 
Failed to open \EFI\BOOT\grubx64.efi - Not Found 
Failed to load image \EFI\BOOT\grubx64.efi: Not Found 
start_image() returned Not Found 

How could this occur because of an update and how to fix this? 

What I tried is booting from centos usb, chroot to /mnt/sysimage and gave command: 
efibootmgr --create --label CentOS --disk /dev/sda1 --loader "\EFI\centos\shim.efi" 

This gave no EM, but booting remains impossible. 

I have to say I don't know whether to try grub2 of efibootmgr commands. 

Any help would be very much appreciated. 

Greetings, J.