OK, reply to myself, but just to document the solution. In the end Daniel assisted me and we found a solution. Using the normal HA scrape sensor, it turned out Multiscrape was not necessary. The scrape itself gets a json, from which multiple sensors can be read out. That was my misunderstanding.
Now I use the following scrape sensor, really easy:
rest:
- resource: 'http://192.168.2.138/get.json?f=$.status.*'
scan_interval: 10
sensor:
- name: Vloerverwarming Modus
value_template: '{{ value_json.status.outputs.heater.mode }}'
- name: Vloerverwarming Pomp
unit_of_measurement: "%"
value_template: '{{ value_json.status.outputs.pump.speed }}'
- name: "Vloerverwarming Klep"
unit_of_measurement: "%"
value_template: '{{ value_json.status.outputs.valves.8.state }}'
- name: "Vloerverwarming Temperatuur In"
unit_of_measurement: "°C"
value_template: '{{ value_json.status.inputs.max.temperature }}'
- name: "Vloerverwarming Temperatuur Uit"
unit_of_measurement: "°C"
value_template: '{{ value_json.status.inputs.return.temperature }}'
Thanks again to Daniel for helping me out.
Just to be clear, something strange did happen in the reply from the UMR2 and the processing by Multiscrape. But since Daniel does not have that controller, he cannot reproduce it.