Systemd complained about "Unknown user 'systemd-network'":

systemctl status systemd-tmpfiles-setup -l
��� systemd-tmpfiles-setup.service - Create Volatile Files and Directories
   Loaded: loaded (/usr/lib/systemd/system/systemd-tmpfiles-setup.service;
static; vendor preset: disabled)
   Active: failed (Result: exit-code) since Tue 2018-04-03 09:48:07 CEST; 9min
ago
     Docs: man:tmpfiles.d(5)
           man:systemd-tmpfiles(8)
 Main PID: 1230 (code=exited, status=1/FAILURE)

Apr 03 09:48:07 linux-g3a6 systemd[1]: Starting Create Volatile Files and
Directories...
Apr 03 09:48:07 linux-g3a6 systemd-tmpfiles[1230]:
[/usr/lib/tmpfiles.d/systemd.conf:19] Unknown user 'systemd-network'.
Apr 03 09:48:07 linux-g3a6 systemd-tmpfiles[1230]:
[/usr/lib/tmpfiles.d/systemd.conf:20] Unknown user 'systemd-network'.
Apr 03 09:48:07 linux-g3a6 systemd-tmpfiles[1230]:
[/usr/lib/tmpfiles.d/systemd.conf:21] Unknown user 'systemd-network'.
Apr 03 09:48:07 linux-g3a6 systemd[1]: systemd-tmpfiles-setup.service: Main
process exited, code=exited, status=1/FAILURE
Apr 03 09:48:07 linux-g3a6 systemd[1]: Failed to start Create Volatile Files
and Directories.
Apr 03 09:48:07 linux-g3a6 systemd[1]: systemd-tmpfiles-setup.service: Unit
entered failed state.
Apr 03 09:48:07 linux-g3a6 systemd[1]: systemd-tmpfiles-setup.service: Failed
with result 'exit-code'.


But at the end of /etc/passwd you see:

...
+::::::
systemd-network:x:465:465:systemd Network Management:/:/sbin/nologin

Maybe, this was added while testing rmt.