Can't use attributes from Tasks API in Flow Designer

[Ove-Tolk]

Hi all,

I’m using a form that creates a callback task in Cisco Webex Contact Center via the API (POST /v1/tasks). As part of the request, I’m passing customer data (e.g. customerNameNeu) using the attributes field.

The request body im using:
{"destination": "00173xxxxxxxx",
"entryPointId": "ecaa1072-c566-4440-xxxx-xxxxxxxxx",
"origin": "+49xxxxxxxxxxx,

 "attributes": {

    "customerNameNeu": "Test NAME"

  },

  "outboundType": "EXECUTE_FLOW",

  "mediaType": "telephony"

}

The attributes which are passed to the flow, cant be used in that flow. 
I added the variable in the flow designer (have a look at the screenshots).

Passing variables using CALLBACK as outboundType isn't working as well. 

Do you see any problem in my configuration? 

According to this vidcast of Cisco it should work:
Vidcast - Preferred Agent Callback for Webex Contact Center

Thanks in advance!

Best regards

Ove