GRUB can't chainload Windows under Secure Boot

[Giovanni Santini]

Good evening,
I have a strange issue with GRUB.
I have multiple OSes on my laptop and one of them is Windows.
GRUB detects it properly and can chainload it...
When Secure Boot (from now on SB) is disabled.

When SB is on, I receive a pretty strange error message, similar to:

/EndEntire
file path:
/ACPI(yadda)/PCI(yadda)/Sata(0,0,0)/HD(yaddayadda)/File(\EFI\Microsoft\Boot)/File(bootmgfw.efi)/EndEntire
error: cannot load image.

Some peculiar notes:
- GRUB can boot behind SB. I can the Secure Boot EFI variables and they
report a successful secure boot.
- The same executable (bootmgfw.efi) is loaded properly from the UEFI
firmware.
- GRUB, to boot behind SB, was setted up to work with Preloader and
HashTool.

I can provide any kind of logs you can think of.
The only thing I am missing are ideas of what to do for fixing it ;(

I hope to have some feedback soon!
Thanks in advance.

-- 
Giovanni Santini
My blog: http://giovannisantini.tk
My code: https://git{hub,lab}.com/ItachiSan
My GPG: 2FADEBF5