Hass.io as Alexa https endpoint

Configuration
1

Hello together,
I tried to setup ha on hass.io to handle alexa skills. I found several tutorials about it. Most of them worked. Some are outdated and I heared there were changes in 0.50. I tried everything. But I can’t get over one problem. The skill seems to work and ha seems to reponse, but alexa always tells me: “The remote endpoint could not be called, or the response it returned was invalid.”. If I check the response with postman, alls seems to be ok. This is the json I get via postman:

{
    "response": {
        "card": {
            "content": "I have turned on the lights.",
            "title": "Lights Card",
            "type": "Simple"
        },
        "outputSpeech": {
            "text": "I did the work, yay.",
            "type": "PlainText"
        },
        "shouldEndSession": true
    },
    "sessionAttributes": {},
    "version": "1.0"
}