@altoid, @chris2be8
These tests don't change the result. NetworkManager resets the resolv.conf file to the old value at restart.
It must be the ISP's problem. I'm actually on holiday, hence at another location.

No further analysis is required. I will report back next week.
Thank you all, greetings, Andre

nslookup shows:

andre@yokohama:~$ nslookup deb.devuan.org
Server:		192.168.1.1
Address:	192.168.1.1#53

Non-authoritative answer:
deb.devuan.org	canonical name = deb.rr.devuan.org.
Name:	deb.rr.devuan.org
Address: 190.64.49.124
Name:	deb.rr.devuan.org
Address: 160.16.137.156
Name:	deb.rr.devuan.org
Address: 185.178.192.43
Name:	deb.rr.devuan.org
Address: 198.58.118.8
Name:	deb.rr.devuan.org
Address: 125.228.189.120
Name:	deb.rr.devuan.org
Address: 94.16.114.15
Name:	deb.rr.devuan.org
Address: 185.236.240.103
Name:	deb.rr.devuan.org
Address: 106.178.112.231
Name:	deb.rr.devuan.org
Address: 5.161.180.234
Name:	deb.rr.devuan.org
Address: 67.219.104.166
Name:	deb.rr.devuan.org
Address: 202.61.197.17
Name:	deb.rr.devuan.org
Address: 185.183.113.131
Name:	deb.rr.devuan.org
Address: 131.188.12.211
Name:	deb.rr.devuan.org
Address: 147.78.194.22
Name:	deb.rr.devuan.org
Address: 195.85.215.180
Name:	deb.rr.devuan.org
Address: 103.146.168.12
Name:	deb.rr.devuan.org
Address: 95.216.15.86
Name:	deb.rr.devuan.org
Address: 200.236.31.1
Name:	deb.rr.devuan.org
Address: 46.4.50.2
Name:	deb.rr.devuan.org
Address: 130.225.254.116
Name:	deb.rr.devuan.org
Address: 141.84.43.19
Name:	deb.rr.devuan.org
Address: 2a01:4f9:2a:fa9::2
Name:	deb.rr.devuan.org
Address: 2801:82:80ff:8000::2
Name:	deb.rr.devuan.org
Address: 2a01:4f8:140:1102:2b76:955d:b48f:bdf3
Name:	deb.rr.devuan.org
Address: 2001:878:346::116
Name:	deb.rr.devuan.org
Address: 2001:4ca0:4300::1:19
Name:	deb.rr.devuan.org
Address: 2800:a8:c001::a
Name:	deb.rr.devuan.org
Address: 2001:e42:102:1704:160:16:137:156
Name:	deb.rr.devuan.org
Address: 2a03:4000:28:24c::
Name:	deb.rr.devuan.org
Address: 2a0d:eb00:8006::acab
Name:	deb.rr.devuan.org
Address: 240b:10:f00:1b00::240
Name:	deb.rr.devuan.org
Address: 2a01:4ff:f0:dd3a::1
Name:	deb.rr.devuan.org
Address: 2401:c080:2000:229e:4b70:fe82:36ed:f788
Name:	deb.rr.devuan.org
Address: 2a03:4000:59:123:68cc:97ff:fee1:c81
Name:	deb.rr.devuan.org
Address: 2001:638:a000:1021:21::1
Name:	deb.rr.devuan.org
Address: 2a0a:e5c0:10:3::6eeb
Name:	deb.rr.devuan.org
Address: 2a01:9e40::180
Name:	deb.rr.devuan.org
Address: 2407:b6c0::12

andre@yokohama:~$

traceroute never responds:

andre@yokohama:~$ traceroute deb.devuan.org

As I wrote: every address responds well with the only exception of deb.org.devuan.
I can live with the situation, but I will try again next weekend when I will be at another place, physically.
Then I will report back. Thanks to you all for your good will.

Oh, after a long while traceroute came back with some answer:

andre@yokohama:~$ traceroute deb.devuan.org
deb.devuan.org: Name or service not known
Cannot handle "host" cmdline arg `deb.devuan.org' on position 1 (argc 1)
andre@yokohama:~$ 

funny, isn't it?

Such things can, but should not happen, in cases where you have a system-crash while editing a file, or at least when it was opened for writing.
Since you have an ext3 filesystem, which is the first release oft ext filesystems that is a journaling filesystem by default, it's less likely to happen. But it's not guaranteed in all situations. fsck and other repair tools can wipe such accident-files. Check in /lost+found directory of the concerned filesytemm. It's the place for recovered blocks.

And: tools that sync or snpashot filesystems for backup, can delete files in the backup volume if the file is no longer present in the source filesystem. Check the options.

Man pages and a bit of study of the tools and systems used can well remove some spookiness.
Good luck.

PS: pulling the power-plug or removing a mounted external disk suddenly can be considered system-crashes as well.

rolfie's last comment is absolutely right and perfect.
I would just add, as a measure to make sure it works: erase the disk beforehand and let the installer do a fresh partitioning. Meaning the installer will decide what disk-layout architecture to choose. Most probably it will be GPT.
You can do it manually, if you know what you do and what you should do.

@kapqa
please be reminded that Ubuntu is a systemd based distribution and Devuan is not.
There are important differences here. Devuan has replaced all systemd-dependent components with very reasonable alternatives.
So please read the installation instructions, readme-s, and all related important text.

To use hibernation, the swap-partition must be at least a bit bigger than your RAM in the computer.
Elogind is just a replacement for the systemd-related logind - afaik.
I can tell you if you perform a standard installation (devuan desktop) you will most certainly get a well working system.
Also: I had very good results every-time with Devuan - OpenRC - Cinnamon or XFCE, on most of the hardware I tested.
And again: RTFM! (Read The Fascinating Manuals)

Hello,
It's technically possible. But:
I always used a dedicated partition mounted on /server for such uses.
For example :

/server/web/
/server/web/http/
/server/web/python/
/server/ftp/
/server/whatever-db/

etc,
That way your web services will never fill up your root filesystem, and it was usual to do so with all kinds of Enterprise Linux Distros.
But my experience with that is 10+ years old. Possibly new habits did appear?
Just my considerations..... hope it is meaningful.

PS: in fact, it's just another name, but it's on its own partition or filesystem.

It's bizarre, I have a brand new Dell Latitude 5540 notebook computer with such a M2 "PCIe Gen4 x4 NVMe," SSD drive, and GRUB on this device was no problem.
I have 2 Linux systems in dual-boot mode, the main one is Devuan 5, the other Linux Mint 21.3 (just for support to others).
When received, I directly set up the UEFI-BIOS to default values and switched off the "Secure Boot" option. Then I wiped the disk and installed both Linux systems without any snag or hitches.
I believe that your computer came with Windows pre-installed in UEFI-Secure-Boot mode. If possible, try to re-install Windows with Secure-Boot disabled in UEFI setup first, and then install your Devuan behind it. Secure-Boot is a free ticket to hell. It binds you firmly to Microsoft or at least to big-money corporate Linux.
In my case, I installed a Windows in a VM on my Devuan installation (KVM) for just that one and only damned windows-application (audio related) I really need.
I can't advise you any better.
Good luck.

@chris2be8
Switzerland.
But firefox-esr was definitely the problem. Firefox-"fresh" really resolves the issue.

Yes, indeed,
I just now see your 2 messages.... but yes, I encountered the same problem with transparency and could do nothing about it.
Now, thank you for the hint with compton, I will try it the next time I configure a box with xfce.
On my work-horses (daily machines) I use lightdm and Cinnamon, and I'm very happy with it.
But lightdm with xfce looks very appealing to me, specially when the panel transparency works again. Thank you.

A true word.... Devuan may consider to just offer something like firefox-fresh or libreoffice-fresh in their repos.
It happened a few times to me that some crucial (to us) WEB-sites suddenly stop to work because of firefox-esr.
I had to download firefox from mozilla.org and install it their way, which works very well. And then, I still prefer firefox to every other browser there is.
Just a thought.

just install ntpsec:

$ sudo apt install ntpsec

It does all you need, syncs your clock with debian's ntp pool.
Check with

$ sudo ntpq -p

Just if you wonder: Configfile = /etc/ntpsec/ntp.conf