Youtube Data API v3 for IOS: Login Requierd 401 Error when posting

I have some troubles with the Youtube Data API. I just want to rate a video. What I did:

Login the User with his google account and with necessary scopes. Then call the HTTP Request to rate the video

func setRating(objectID: String, completionHandler:(error:NSError?) -> Void ) {

    let url = "https://www.googleapis.com/youtube/v3/videos/rate?id=(objectID)&rating=like&key=(self.kAPI)".convertToURL()
    Alamofire.request(.POST, url, encoding: .JSON)
        .validate()
        .response { request, response, data, error in
            if response == nil {
                print("No Response: (error)")
                completionHandler(error: error)
                return
            }
            if error != nil {
                print("There is an error: (error)")
                ErrorHandler.sharedInstance.showYoutubeLoginError()
                completionHandler(error: error)
                return
            }
            completionHandler(error: nil)
    }
}

I get the 401 Error with this request. The funny part is, that worked some time ago. But I don't know how and why. Everything is correct with the API Key. It works with GET Requests. Is there something that I'm missing?