The machine is a new HP 15-db0007no laptop with AMD/ATI graphics which
needs firmware-amd-graphics to work well.  My older HP/AMD/ATI laptop
used to start in some framebuffer mode, which made further work possible,
firmware installs etc.  I guess that the installer and Grub works with
some fb, but I couldn't copy them, fiddling with xorg.conf, which used
to work in earlier days.

Anyhow, I hope that not many other newcomers will copy my bad luck :
choosing the dvd before the Live iso ( partly because I guessed the
installation would work better from the dvd, I use different partitions ).
Then learned that the Live version had the graphical firmware aboard,
and furthermore a failsafe Grub entry, both of which aren't found on
the dvd iso !!  I'm sorry, but must consider this a serious mistake
from the honorable Devuan crew !  At the very least, put in the failsafe
entry into all install media !

( I first wrote a text with tips for other unlucky users, to maybe post
in the installation, hardware or doc pages, but You maybe see my
problems as too unique ..? )

Anyhow : from now on always Devuan !!