Dear all,

the issue is solved.

During research for Ralphs questions I realized my UEFI/BIOS was severly out
of date (I had thought this would have been done during refurbishing of the
device). After updating from a 2016 UEFI to one from 2020 (many thanks to HP
for still supporting such a rather old device) Grub works from Devuan.

Still it would be interesting, why Debian worked even with the old bios, but I
guess I have neither EFI knowledge nor enough motivation to find out.

Thanks a lot, all of you and a happy system-d free time!

Kind regards,
Jonathan