Youtube activities endpoint not returning any items

I've seen some other users ask this question, but really haven't found any answers... Similar to what was already asked in this thread, I have a problem with retrieving items from the Youtube API activities endpoint. My request is as follows:

GET https://youtube.googleapis.com/youtube/v3/activities?part=snippet%2CcontentDetails&channelId=[channelId]&key=[YOUR_API_KEY]  --header 'Authorization: Bearer [YOUR_ACCESS_TOKEN]' \  --header 'Accept: application/json' \  --compressed

And I get responses with HTTP status 200 OK but zero items:

{"kind": "youtube#activityListResponse","etag": "ETAG","items": [],"pageInfo": {"totalResults": 1,"resultsPerPage": 25  }}.

And I tried with my own account and an account with public data allowed, so I should by all logic get a response with some items. Anyone who solved this already?