Comment # 2 on bug 1234612 from greg mott

also nagios won't start with the packaged
/usr/lib/systemd/system/nagios.service

this works:
[Unit]
Description=Nagios
After=syslog.target network.target

[Service]
# for details of the additional system hardening configuration please see
# https://en.opensuse.org/openSUSE:Security_Features#Systemd_hardening_effort
ProtectSystem=full
ProtectHome=true
ProtectHostname=true
ProtectControlGroups=true
ProtectKernelModules=true
ProtectKernelTunables=true
RestrictRealtime=true
# end of system hardening
Type=simple
User=nagios
Group=nagios
ExecStart=/usr/sbin/nagios    /etc/nagios/nagios.cfg
ExecReload=/usr/sbin/nagios -v /etc/nagios/nagios.cfg
ExecReload=kill -HUP $MAINPID

[Install]
WantedBy=multi-user.target