Still trying… and fighting 
Ok, there is some progress.
After fix services nagios-nrpe-server (post above) I tried install Nagios again. As expected there was an error with nagios service and service did not run:
Administrator@srv1:~$ sudo service nagios restart
Job for nagios.service failed because the control process exited with error code.
See “systemctl status nagios.service” and “journalctl -xe” for details.
Administrator@srv1:~$ sudo service nagios status
● nagios.service - LSB: nagios host/service/network monitoring and management system
Loaded: loaded (/etc/init.d/nagios; generated; vendor preset: enabled)
Active: failed (Result: exit-code) since Fri 2019-01-11 08:28:56 CET; 6s ago
Docs: man:systemd-sysv-generator(8)
Process: 7499 ExecStop=/etc/init.d/nagios stop (code=exited, status=0/SUCCESS)
Process: 8928 ExecStart=/etc/init.d/nagios start (code=exited, status=1/FAILURE)
CPU: 350ms
sty 11 08:28:56 srv1 nagios[8928]: version of Nagios, you should be aware that some variables/definitions
sty 11 08:28:56 srv1 nagios[8928]: may have been removed or modified in this version. Make sure to read
sty 11 08:28:56 srv1 nagios[8928]: the HTML documentation regarding the config files, as well as the
sty 11 08:28:56 srv1 nagios[8928]: ‘Whats New’ section to find out what has changed.
sty 11 08:28:56 srv1 nagios[8928]: errors in config! … failed!
sty 11 08:28:56 srv1 nagios[8928]: failed!
sty 11 08:28:56 srv1 systemd[1]: nagios.service: Control process exited, code=exited status=1
sty 11 08:28:56 srv1 systemd[1]: Failed to start LSB: nagios host/service/network monitoring and management system.
sty 11 08:28:56 srv1 systemd[1]: nagios.service: Unit entered failed state.
sty 11 08:28:56 srv1 systemd[1]: nagios.service: Failed with result ‘exit-code’.
I checked details:
Administrator@srv1:~$ sudo journalctl -xe
sty 11 08:28:56 srv1 systemd[1]: Starting LSB: nagios host/service/network monitoring and management system…
(…)
sty 11 08:28:56 srv1 nagios[8928]: Reading configuration data…
sty 11 08:28:56 srv1 nagios[8928]: Read main config file okay…
sty 11 08:28:56 srv1 nagios[8928]: Error: Could not open config directory ‘/etc/nagios/conf.univention.d/contacts’ for reading.
(…)
For some reason permissions were 700 instead of 755 for /etc/nagios.conf.univention/, so I fixed it manually.
I restarted nagios service and all semmed to be ok:
Administrator@srv1:~$ sudo service nagios status
● nagios.service - LSB: nagios host/service/network monitoring and management system
Loaded: loaded (/etc/init.d/nagios; generated; vendor preset: enabled)
Active: active (running) since Fri 2019-01-11 08:39:13 CET; 1s ago
Docs: man:systemd-sysv-generator(8)
Process: 9450 ExecStop=/etc/init.d/nagios stop (code=exited, status=0/SUCCESS)
Process: 9482 ExecStart=/etc/init.d/nagios start (code=exited, status=0/SUCCESS)
Tasks: 8 (limit: 4915)
Memory: 3.5M
CPU: 366ms
CGroup: /system.slice/nagios.service
├─9525 /usr/sbin/nagios -d /etc/nagios/nagios.cfg
├─9526 /usr/sbin/nagios --worker /var/lib/nagios/rw/nagios.qh
├─9527 /usr/sbin/nagios --worker /var/lib/nagios/rw/nagios.qh
├─9528 /usr/sbin/nagios --worker /var/lib/nagios/rw/nagios.qh
├─9529 /usr/sbin/nagios --worker /var/lib/nagios/rw/nagios.qh
├─9530 /usr/sbin/nagios --worker /var/lib/nagios/rw/nagios.qh
├─9531 /usr/sbin/nagios --worker /var/lib/nagios/rw/nagios.qh
└─9532 /usr/sbin/nagios -d /etc/nagios/nagios.cfg
sty 11 08:39:13 srv1 nagios[9525]: WARNING: The retry_check_interval attribute is deprecated and will be removed in future versions. Please use retry_interval instead.
sty 11 08:39:13 srv1 nagios[9525]: WARNING: The normal_check_interval attribute is deprecated and will be removed in future versions. Please use check_interval instead.
sty 11 08:39:13 srv1 nagios[9525]: WARNING: The retry_check_interval attribute is deprecated and will be removed in future versions. Please use retry_interval instead.
sty 11 08:39:13 srv1 nagios[9525]: WARNING: The normal_check_interval attribute is deprecated and will be removed in future versions. Please use check_interval instead.
sty 11 08:39:13 srv1 nagios[9525]: WARNING: The retry_check_interval attribute is deprecated and will be removed in future versions. Please use retry_interval instead.
sty 11 08:39:13 srv1 nagios[9525]: WARNING: The normal_check_interval attribute is deprecated and will be removed in future versions. Please use check_interval instead.
sty 11 08:39:13 srv1 nagios[9525]: WARNING: The retry_check_interval attribute is deprecated and will be removed in future versions. Please use retry_interval instead.
sty 11 08:39:13 srv1 nagios[9525]: WARNING: The normal_check_interval attribute is deprecated and will be removed in future versions. Please use check_interval instead.
sty 11 08:39:13 srv1 nagios[9525]: WARNING: The retry_check_interval attribute is deprecated and will be removed in future versions. Please use retry_interval instead.
sty 11 08:39:13 srv1 nagios[9525]: Successfully launched command file worker with pid 9532
But it is not the end of problems…
Nagios is active and I can log into but there on only errors:
|[srv1.local.intranet
||[UNIVENTION_CUPS]|CRITICAL|2019-01-11 08:58:13|0d 0h 15m 24s|5/5|CHECK_NRPE: Error - Could not connect to 10.10.84.5: Connection reset by peer|
||[UNIVENTION_DISK_ROOT]|CRITICAL|2019-01-11 08:58:13|0d 0h 10m 24s|10/10|CHECK_NRPE: Error - Could not connect to 10.10.84.5: Connection reset by peer|
||[UNIVENTION_DNS]|CRITICAL|2019-01-11 08:49:41|0d 0h 18m 56s|10/10|CHECK_NRPE: Error - Could not connect to 10.10.84.5: Connection reset by peer|
||[UNIVENTION_JOINSTATUS]|CRITICAL|2019-01-11 08:59:13|0d 0h 0m 24s|1/1|CHECK_NRPE: Error - Could not connect to 10.10.84.5: Connection reset by peer|
||[UNIVENTION_LOAD]|CRITICAL|2019-01-11 08:55:25|0d 0h 19m 12s|1/1|CHECK_NRPE: Error - Could not connect to 10.10.84.5: Connection reset by peer|
||[UNIVENTION_NSCD2]|CRITICAL|2019-01-11 08:47:55|0d 0h 13m 42s|2/2|CHECK_NRPE: Error - Could not connect to 10.10.84.5: Connection reset by peer|
(…)
There is also an issue that there is dc1 with nagios support checked on UCM but in Nagios Core it is visible as host but there are not any services active on it.