I just have written an answer that the forum refused to let me publish
(access denied)...
Anyway, here it is:
https://stephentanner.com/restoring-grub-for-an-encrypted-lvm.html
I took it from here:
https://trisquel.info/en/forum/netinsall-tty-apt-get-update-not-working#comment-98813
The idea is to start a livecd or liveusb, open the CLI, then type a bunch of
commands in order to access your encrypted disk.
From there, you need to know more about vmlinuz and how to replace it or
rename it or whatever makes it work.
Hence the links I've posted above (I don't know if they precisely apply to
your problem though).
Also this explains a bit more: https://en.wikipedia.org/wiki/Vmlinux
No problem, I know how irritating it is to make Libreboot work. My case was
special, but it took me 6 months because of a damn bug: impossible to boot
when the cd drive is inside the Ultrabase.
But that's as far as I can go right now, you need to try chrooting (just
follow the instructions and adapt them if needed), and you need to find out
what to do with vmlinuz.
I'll keep trying to help if I can though.
Just stay calm, be patient, and never give up :)
