Sending Steam Tradeoffer - 400 Null Response

Question

Im trying to send a Steam Tradeoffer in Python, but i always get a 400 - null response. I am aware, that there are some similar questions, but none of these got the "null response" issue resolved. Has someone an Idea, why i always get the 400 response?

My Https request looks like this:

requests.post("https://steamcommunity.com/tradeoffer/new/send", headers=headers, cookies=cookies, data=data)

with this Form data:

{
  "sessionid": "XXXX",
  "serverid": 1,
  "partner": "partner_id",
  "tradeoffermessage": "",
  "json_tradeoffer": {
    "newversion": true,
    "version": 2,
    "me": {
      "assets": [
        {
          "appid": 730,
          "contextid": "2",
          "amount": 1,
          "assetid": "XXXXX"
        }
      ],
      "currency": [],
      "ready": false
    },
    "them": {
      "assets": [],
      "currency": [],
      "ready": false
    }
  },
  "captcha": "",
  "trade_offer_create_params": "tradeOfferToken"
}

this Header:

{
  "Referer": "https://steamcommunity.com/tradeoffer/new/?partner=XXXX&token=XXX"
}

Cookies:

cookies = {
        "sessionid": "XXXXXXXXXXXXXXXXXXXXX",
        "steamLoginSecure": "XXXXXXXXXXXXXXXXXXXXXXXXXX",
    }

My json_tradeoffer is this:

{
  "newversion": true,
  "version": 2,
  "me": {
    "assets": [
      {
        "appid": 730,
        "contextid": "2",
        "amount": 1,
        "assetid": "XXXXX"
      }
    ],
    "currency": [],
    "ready": false
  },
  "them": {
    "assets": [],
    "currency": [],
    "ready": false
  }
}