Oh ever ranting steve_v,
does it ever come to your mind that this is a forum to help people seeking advice?
You help nobody with your outbursts but poison a nice and good community and forum instead.
And what you shout about is not even always correct.
So if you can help, then help.

Considering all that information.....
why not try to do a proper system setup (BIOS/Firmware) and reset ALL settings to default values?
If you truly know about your specifics, you can make some changes after a restart an re-entering the system setup.

Then, start your system with the external USB-stick containing the Devuan installer image.
Go through all steps, and when it comes to the partition setup: let the setup-program erase the disk completely and install the system. I've never seen a case where that doesn't work.
Should you wish to modify the partition setup, you can restart the installer (reboot) and make your modification of the partition-scheme then.
Usually it's okay to have one / (root) partition of at least 50GB, an adequate swap partition (1-2 times the RAM size), and the rest may be /home or a /home and some other partition like /srv.

The installer knows how to figure out the disk-layout, either MBR or GPT. (BIOS-mode or EFI-mode)
Never ever use things in BIOS setup like "secure boot" or "TPM", it brings nothing but grief.

The reason I recommend all this: who knows what that former Windows setup has done to your beautiful computer.

@all
Thank you for your ideas!
Your answers are helpful and I test FF again with your recommendations, in parallel with testing waterfox.
Have a nice weekend!

Have a look in your repos for the packages:
hplip
hplip-gui
(incl. all dependencies)

If there, install them and start hplip-gui.
Configuring your printer should be child's play then.
That works well with devuan, debian, linuxmint, ubuntu etc.

Sure, rolfie is right.
The computer's HW clock should be set to UTC. And the timezone (TZ) should be selected as to the standards.
When DST patterns change for some time-zones, the corresponding patches will be distributed. Another reason to update regularly.

Roberto,
yes, there is a solution to that problem:
Its the NetworkManager version used in Excalibur.
Look at this thread:
https://dev1galaxy.org/viewtopic.php?pid=59374#p59374

The workaround is (from steve_v):

I hope that may be a temporary solution for you too.
I guess there will be a newer or patched network-manager package in some time, possibly a new point-release or a patch.
Good luck.

I think I marked the topic as "solved" to early.
With the release of Devuan 6 Excalibur, anacron in combination with OpenRC still causes delays at shutdown - sometimes. Not every-time but still sometimes.
The problem can be worked around by disabling anacron (rc-update delete anacron) or by switching to sysvinit.

steve_v:

I am blaming NO one!!
I just want people to avoid the trap of such a case!
I'm no politician and no flamer, so please stay on the subject.
Anyway, you gave me the good hint earlier, thanks for that.

To steve_v, ralph.ronnquist, fsmithred:

There is news about our config NetworkManager-OpenRC-Cinnamon: the differences between Devuan Excalibur and Artix.
See the differences in the following reports I have just written:
Excalibur:
https://pastebin.com/4CcBTkEV

Artix:
https://pastebin.com/3KuU9mUy

In the Artix test-install (~6 month old), there is a additional file in /etc/NetworkManager/dispatcher.d/
-rwxr-xr-x 1 root root 1213 May 14  2021 10-openrc-status

And the version of networkmanager is networkmanager 1.54.1-1
The result is that NFS mounts in /etc/fstab are respected at boot time.
I have the additional file here should you be interested.

I will send the same information to the devuan networkmanager maintainer.
Have a lovely Sunday. Greetings, André.

Fsmithred,

I have just done that. Thank you.
Bugreport sent.

Shall I mark the case as solved?

Thank you for your suggestions and help, to all!

@steve_v
You are true hero, you have nailed it.
I did as you suggested and placed that script /etc/NetworkManager/dispatcher.d/01-ifupdown to the right place and rebooted. And sure enough: BINGO.
Thank you for that precious hint.

@ralph.ronnquist
I did the tests on a spare machine, doing a fresh-install with the excalibur-netinstall-iso and running the the devuan installer's first option: install
Test 1:  Excalibur - Cinnamon - OpenRC
Test 2:  Excalibur - Cinnamon - sysvinit
In both cases I just edited the hosts file, the fstab, created the mountpoints and rebooted. In both cases NFS was not mounted.

Copying the famous file /etc/NetworkManager/dispatcher.d/01-ifupdown from my local server (daedalus)  to the test machine did the trick..
BTW: Editing the /etc/rc.local file didn't change a thing.

Would you like me to write a simple procedure (a text file) how to fix that problem? Then, where could I send it for inclusion to the release readme file?

Unfortunately I'm not able to fix the networkmanager package.

Once again, to all of you: Thank you for your help.
Sincerely, Andre (andre4freedom)

@steve_v : Thank you.
I have tried my luck to upload some files. You can find them here (hopefully):

(dmesg):
https://pastebin.com/V3iQa77z

(fstab):
https://pastebin.com/HiGjWMAj

(hosts):
https://pastebin.com/hX8evCQY

(syslog):
https://pastebin.com/UvuWqyVw

(boot-log):
https://pastebin.com/s6tumGik

Thank you for volunteering into that mystery.
Sincerely, Andre4Freedom