Facebook graph api page post with multiple photos

I am trying to publish a page post with multiple photos via the Facebook's graph api.

Currently from the docs and another question here it's stated that the photos should be uploaded separately and then publish the post with attached_media parameter.

The photos are uploaded fine without any problem and I get their IDs. The issue is that the request for publishing the post gives:

{
  "error": {
  "message": "An unknown error has occurred.",
  "type": "OAuthException",
  "code": 1,
  "fbtrace_id": "SOME TRACE ID"
}

If I remove the attached_media parameter from the request the post is published fine.. Any Idea what can be the issue?

Specially for CBroe here are the reqiests: For the photo upload:

endpoint: /{page-id}/photos

payload:

{
  "url": "some-image-url",
  "caption": "Some image caption",
  "published": false
}

endpoint: /{page-id}/feed

payload:

{
  "message": "Some post message",
  "published": false,
  "attached_media": [
    {"media_fbid": "PHOTO_ID_RETURNED_FROM_THE_ABOVE_REQUEST"}
  ]
}

The payload is send as json to the endpoint. This request works only if I remove the attached_media parameter.