I've encrypted my /boot partition with dm-crypt in a luks container. I've added a strong password to the slot #0 of the luks container.
Now I can open the container from a live CD successfully but not from Grub.
I tried to put a weak password instead (4 letters) and it works with Grub.
My strong password is 20 characters long, with letters, digits and punctuation in it.
It's not the first time I'm doing this kind of setup, but I've never encountered such an issue before.
EDIT: I use the default keymap.