Bluesound volume control

My goal is to get a volume slider to control the volume of my powernode (and slaves). I have looked into this topic but I can’t get it to work.

What I have done so far is:
I created a volume channel (number) under my existing Bluesound thing. Filled out the command extension with Volume?level=%2$s (taken from the previous post). After that I created a volume item (dimmer) and linked it to the volume channel.

I am not able to control the volume right now. The slider shows changing volume numbers.
With the command in my browser I can put the volume louder. With -2 at the end the volume will get softer.

This is the code from the channel.

UID: http:url:5651c472bb
label: Bluesound
thingTypeUID: http:url
  authMode: BASIC
  ignoreSSLErrors: false
  delay: 0
  stateMethod: GET
  refresh: 2
  commandMethod: GET
  contentType: application/xml
  timeout: 3000
  bufferSize: 2048
  - id: Status
    channelTypeUID: http:number
    label: Status
    description: ""
      mode: READWRITE
      stateExtension: Status
  - id: Volume
    channelTypeUID: http:number
    label: Volume
    description: ""
      commandExtension: Volume?level=%2$s

And Item

label: Volume
type: Dimmer
category: ""
groupNames: []
groupType: None
function: null
  - Point

This is the warning from the frontail viewer.

Requesting '' (method='GET', content='null') failed: 404 Not Found

I also used this API document from Bluesound.

At this moment I am stuck and any help is appriciated.