EFI boot: 'could not prepare Boot variable: Invalid argument'

[post@thomasdanckaert.be]

Hi Guix-helpers,

I've just switched to a new system, but as of the 4th generation, guix 
system reconfigure fails to install the bootloader.  This is on guix 
commit 3e25944 (but that was also the case when guix system reconfigure 
still worked, I believe).  I fear my system is currently unbootable.

The error seems to come from efibootmgr, which complains about
'Could not prepare Boot variable: Invalid argument'

The failing command, if I'm not mistaken, is
/gnu/store/k1z2szmyvi1na7vzv8vnxmpb51nf4aw9-efibootmgr-17/sbin/efibootmgr 
-c -d /dev/sda -p 1 -w -L Guix -l \EFI\Guix\grubx64.efi.

and I can't find anything wrong with it.  I've also tried running the 
same command with the other version of efibootmgr I could find in 
/gnu/store, but that results in the same error.

Running efibootmgr gives me the following output:

BootCurrent: 0000
No BootOrder is set; firmware will attempt recovery
BootFFFF*

which I think implies that my system is currently unbootable?

Does anybody have a guess what can cause this (and what might solve 
it?).

many thanks!

Thomas