Adobe Sign v6 API and Integration Key does not work but v5 does?

Hey there,

I am trying to upload documents to Adobe Sign. However, I am not able to archive this using an Integration Key (granted every permission) and the Adobe Sign v6 API. 

This is my request:

POST /api/rest/v6/transientDocuments HTTP/1.1
Host: api.eu2.adobesign.com
x-api-user: email:*********
Authorization: Bearer ***************************************
Content-Length: 315
Content-Type: multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW
 
------WebKitFormBoundary7MA4YWxkTrZu0gW
Content-Disposition: form-data; name="File-Name"
 
Basic-NDA.pdf
------WebKitFormBoundary7MA4YWxkTrZu0gW
Content-Disposition: form-data; name="File"; filename="/C:/Users/****/Downloads/GFLF.pdf"
Content-Type: application/pdf
 
(data)
------WebKitFormBoundary7MA4YWxkTrZu0gW--

And this is the response I am getting:

{
    "code": "PERMISSION_DENIED",
    "message": "The API caller does not have the permission to execute this operation"
}

However, when I switch from v6 to v5 (using the same Integration Key) everything works fine and the response contains a valid document id.

Is there any reason why my Integratino Key does not work with the v6 API?

Have Integratin Keys been disabled on v6? If so, will v5 API be still be function in the future or will it be tunred off?

Kind regards