Set Customized boot to \EFI\debian\grubx64.efi
I faced the same problem on HP ProBook 4340s with original Windows 8 already overwritten by new Kubuntu 15.04 (I prefer no dual boot). Among other tries, I also tried Boot-Repair (both from HD after its installation and via boot-repair-disk), but that exercise was most probably unnecessary.
What made the trick was the following changes in the BIOS Setup (after computer start push F10 - go to System Configuration - open Boot Options):
1) Check Customized Boot
2) Check off SecureBoot
3) Boot Mode: choose UEFI Hybrid or UEFI Native (I chose UEFI Native)
4) UEFI Boot Order: put Customized Boot to the top
5) Define Customized Boot Option: choose Add + put the setting: \EFI\ubuntu\grubx64.efi
We found the correct path string when opening the GRUB via the Boot from EFI File option in the Fast Boot Options (F9). In the beginning, the options list contained:
OS boot Manager
Boot from EFI File
After addting the Customized Boot option, it was added to the list:
Customized Boot
OS boot Manager
Boot from EFI File