Empty referrer in YouTube API requests

I'm trying to retrieve a channel's video list from the YouTube API. Actually, this functionality is already implemented, but suddenly stopped working.

Here's what I have:

• A GET request to the search endpoint: https://www.googleapis.com/youtube/v3/search;

• Some query params:

  1. part=snippet,id

  2. channelId=UCSJ4gkVC6NrvII8umztf0Ow (just an example)

  3. maxResults=5

  4. order=date

  5. key=[MY_API_KEY]

I guess everything's alright, but for some reason I get this response:

{
  "error": {
    "code": 403,
    "message": "Requests from referer <empty> are blocked.",
    "errors": [
      {
        "message": "Requests from referer <empty> are blocked.",
        "domain": "global",
        "reason": "forbidden"
      }
    ],
    "status": "PERMISSION_DENIED"
  }
}

I really don't know what's wrong, and every request returns the same error.

Can someone help me please? Thanks for the attention!