"HttpPOST" action gives a {"errorCode":"Forbidden"}

I have created Creating Office 365 Connectors for Microsoft Teams successfully. I can send message card to a teams channel. The message card contains an "HttpPOST" action, however when I click button I get an error inside the MS teams apps. The application tries to post to the following https://teams.microsoft.com/api/mt/emea/beta/users/connectors/19:[email protected];messageid=1620161396652/1620161396652/executeAction, however it receives the following error message {"errorCode":"Forbidden"}.

My connector is register succesfully, with the question "Do you want to enable actions on your Connector cards?" set to yes.

Anybody has any ideas?

{
    "@type": "MessageCard",
    "@context": "http://schema.org/extensions",
    "themeColor": "0076D7",
    "summary": "Larry Bryant created a new task",
    "sections": [{
        "activityTitle": "Larry Bryant created a new task",
        "activitySubtitle": "On Project Tango",
        "activityImage": "https://teamsnodesample.azurewebsites.net/static/img/image5.png",
        "facts": [{
            "name": "Assigned to",
            "value": "Unassigned"
        }, {
            "name": "Due date",
            "value": "Mon May 01 2017 17:07:18 GMT-0700 (Pacific Daylight Time)"
        }, {
            "name": "Status",
            "value": "Not started"
        }],
        "markdown": true
    }],
    "potentialAction": [{
        "@type": "ActionCard",
        "name": "Add a comment",
        "inputs": [{
            "@type": "TextInput",
            "id": "comment",
            "isMultiline": false,
            "title": "Add a comment here for this task"
        }],
        "actions": [{
            "@type": "HttpPOST",
            "name": "Add comment",
            "target": "<my url>"
        }]
    }]
}