Problem defining a REST based sensor


I used to have a REST based sensor reading some weather data from a weather station very near to my home. This sensor worked before - however, I installed HA on a new machine, and I deliberately wanted to recreate most of my HA config. And for some reason, I do not get this sensor running again.

Here’s my code in sensors.yaml:

- platform: rest
  name: upb_json_wetter
  resource: <the URL>
  value_template: "OK"
  scan_interval: 300
    - temperatur
    - luftdruck
    - luftfeuchte
    - windgeschwindigkeit
    - windgrad

- platform: template
      friendly_name: "UPB-Wetter - Temperatur"
      value_template: '{{ states.sensor.upb_json_wetter.attributes["temperatur"] }}'
      unit_of_measurement: "°C"
      device_class: "temperature"
      friendly_name: "UPB-Wetter - Luftdruck"
      value_template: '{{ states.sensor.upb_json_wetter.attributes["luftdruck"] }}'
      unit_of_measurement: "hPa"
      device_class: "pressure"
      friendly_name: "UPB-Wetter - Luftfeuchtigkeit"
      value_template: '{{ states.sensor.upb_json_wetter.attributes["luftfeuchte"] }}'
      unit_of_measurement: "%"
      device_class: "humidity"
      friendly_name: "UPB-Wetter - Windgeschwindigkeit"
      value_template: '{{ states.sensor.upb_json_wetter.attributes["windgeschwindigkeit"] }}'
      unit_of_measurement: "km/h"
      friendly_name: "UPB-Wetter - Windrichtung"
      value_template: '{{ states.sensor.upb_json_wetter.attributes["windgrad"] }}'
      unit_of_measurement: "°"

Under Developer tools/States, I can see the five sensors upb_json_wetter_(temperatur|luftdruck|...), but they all are unavailable. However, I can’t see the upper sensor upb_json_wetter. If I open the resource URL, my browser shows the JSON as expected, so that’s apparently not the problem.

Any idea of what I’m doing wrong? How can I check whether the REST call is done? Do I have to do anything in particular to “enable” the REST platform?

Thanks in advance