More new hardware and in this case (and depending on the type/quality of the UPS), a potential source of problems.
eg: while your PSU in you desktop may play nicely with your AC line, smoothing out micro-spikes or simply ignoring them, your new UPS may not be doing the same thing or even be reacting badly.

Its wise thoughts but i do not think that UPS is source of problems.

its ok, its doomseeker(doom game server browser) official repo.

After reboot "empty /var/log/*" folder problem solved - now all content of /var/log/* is at its usual place. OS fully functional. But again, what was that?

You're welcome.

Q:
You mean that it only happened once?
ie: you cannot reliably reproduce it?

Right ...
A desktop.

Looks normal.

There could be an issue with the drive.
ie: new hardware

More new hardware and in this case (and depending on the type/quality of the UPS), a potential source of problems.
eg: while your PSU in you desktop may play nicely with your AC line, smoothing out micro-spikes or simply ignoring them, your new UPS may not be doing the same thing or even be reacting badly.

A good/decent UPS has always been hard to come by ($$$).
Many years ago I ended up deciding that it was much better to go without one that to end up spending more money on a (nother) UPS which could cause problems.

Please remove your new UPS from the equation till we can find out what is going on.

----

Indeed ...

From what I see of the terminal output you posted at imgur, there is at least one thing that is absolutely out of place:

https://debian.drdteam.org   <- this has nothing to do with any Devuan repository.

Seeing that his is a new installation, I suggest that you do a complete reformat of your new HDD and start over, closely following the instructions* the installer provides you on the screen.

* ie: do not screw around the file system as root if you don't know what you are doing.

So ...
In your place I'd do this:

1. disconnect the UPS from your box, you can get back to it once you have a properly working installation. Or get your money back. 
2. download the current Daedalus Live-desktop, burn it to a USB drive and boot from there. (link posted in my previous reply)
3. use the machine for a while/a couple of days and see how it runs.

If things go as you expect them to go, then you can install Devuan Daedalus from the current Daedalus Desktop installer *.iso.

That should get you on your way.

Best,

A.

root@devuan:/home/freeartist-devuan# apt install df
E: Could not open lock file /var/lib/dpkg/lock-frontend - open (2: No such file or directory)
E: Unable to acquire the dpkg frontend lock (/var/lib/dpkg/lock-frontend), are you root?
root@devuan:/home/freeartist-devuan# 

root@devuan:~# sudo dpkg --configure -a
dpkg: unrecoverable fatal error, aborting:
 unable to fill /var/lib/dpkg/updates/tmp.i with padding: No space left on device
root@devuan:~# 

but root is not full of capacity
https://imgur.com/a/oSBCMEY

ok, thanks, i will look.

Hard to say ...  8^°

You are not explaining much here.
Please bear in mind that an incomplete question will only get you incomplete answers.

For starters:

- Is this on a desktop or laptop/netbook, etc.?
- If a laptop/netbook, under AC or battery power?

- At what point does this (sudden reboot) happen?
ie: right after it boots?, after a while using the machine?

- Can you reproduce the problem if you boot from the current Devuan 5 Live-iso?

- Under what ambient conditions?
ie: hot summer / AC turned on?, any machinery running nearby or on the same circuit?

- Exactly what are you doing with the system when this happens?
- Do you see a pattern?
- Any peripherals connected to the system's hardware?

- When did this start to happen?
eg: Any changes in OS (updates/upgrades) hardware / ambient conditions?

Start by checking the last (previous) dmesg output for any warnings.
ie: /var/log/dmesg.0 file

Also check:

/var/log/syslog and /var/log/messages -> stores all global system activity data, including startup messages.
/var/log/auth.log stores all security-related events such as logins, root user actions.
/var/log/kern.log stores kernel events, errors, and warning logs.

Best,

A.