efibootmgr: Cannot translate unix loader path xxx\xxx\xxx to UEFI: No error: 0

Updated Fujitsu Celsius M740 to its lates UEFI firmware today.
After this update, the box won't boot FreeBSD 12-STABLE anymore! With
disabled CSM, the firmware doesn't recognise the boot SSD's freebsd-efi
partition for UEFI boot anymore - which was no problem before.

When trying FreeBSD 13-CURRENT (USB image from 26.12.2018 as of the
snapshot site) I receive a malloc arena error when trying to set boot
vars via efibootmgr utility. So I tried the recent 12-STABLE snapshot
as of 26th December 2018, the same as CURRENT USB Image, and I receive
a weird error:

efibootmgr -c -l /mnt/EFI/BOOT/BOOTX64.EFI -L FreeBSD

efibootmgr: Cannot translate unix loader path
'\mnt\EFI\BOOT\BOOTX64.EFI' to UEFI: No error: 0

What the heck is that?

What does this error mean? No error: 0?

The box is unusable.


Kind regards,

O. Hartmann