Unexpected behaviour with Web Playback SDK

[easdafdsf]

Hello,

Authentication is working and the locally hosted web application both successfully initializes new Spotify.Player(...)  and starts to play the audio.

Requests to /v1/me/player/play return a 204, however, playback is not modified to desired Spotify URI: 
{

   uris: [<track uri>]
}

Also the API returns a 404 ( "Player command failed: No active device found"), if no other Spotify Clients are open. The request contains query parameter device_id of previously created new Playback SDK player instance, which is activated with PUT request (204) to /player endpoint with the following body: 
{
   device_ids: [<newly_created_player_id>],
   play: true
}

Am I missing something in the documentation? Does localhost have affect on the Playback SDK's behaviour? 

Thanks in advance.