Trouble setting up device_tracker

Dear Community,

I have trouble setting up a simple device_tracker using nmap.
The following setup is in the configuration.yaml

device_tracker:
  - platform: nmap_tracker
    hosts: 10.11.12.13-14
    home_interval: 100
    track_new_devices: no
    interval_seconds: 60
    consider_home: 600

Nevertheless I dont’ see anything in the Web UI.
The following lines are in the log file:

2017-12-05 19:17:09 INFO (Thread-25) [homeassistant.components.device_tracker.nmap_tracker] Scanning...
2017-12-05 19:17:12 INFO (Thread-25) [homeassistant.components.device_tracker.nmap_tracker] nmap scan successful

Also the following:

2017-12-05 18:31:32 ERROR (MainThread) [homeassistant.components.device_tracker] Unable to load /home/homeassistant/.homeassistant/known_devices.yaml: Config file not found: /home/homeassistant/.homeassistant/known_devices.yaml

My Setup:
I’m running Home-assistant on a Debian Stretch LXC Container in an Python3 Virtual Env
The Home of the user “homeassistant” and all files in ~homeassistant/.homeassistant belong to the user “homeassistant”.

python-nmap, nmap and arp are installed and /usr/sbin is in the systemd unit file of homeassistant since it is not in the default path (/usr/sbin):

[Service]
Type=simple
User=%i
ExecStart=/srv/homeassistant/bin/hass -c "/home/homeassistant/.homeassistant"
Environment=PATH="$VIRTUAL_ENV/bin:/usr/sbin:$PATH"

Creating the file known_devices.yaml does not help at all. The “Unable to Load” message goes away, but still I dont’ see anything in the Web Ui as I would expect.

Anyway, the device_tracker module is loaded, because I see the device_tracker Domain under “dev-service”. But I’am not able to see devices. Since I’am new to Home Assistant I don’t know where to look further for the devices.