How to solve Stream encountered HTTP error 403 in Python?

I am trying to create a Stream Listener in tweepy to listen for incoming Tweets on a specific page / account.

I followed all the guidelines so far: I have elevated access for the Twitter API, I have fresh regenerated keys with read/write/DM checks, OAuth 1.0a turned on, triple checked the keys used in the script... and with all of those... still 403 Error.

 class MyStreamListener(tweepy.Stream):

    def on_status(self, status):
        print(status.id)
        if not status.text.startswith("RT ") and status.in_reply_to_screen_name is None:
            print('https://twitter.com/' + username + "/status/" + str(status.id))


# create stream and filter on a searchterm
twitterListener = MyStreamListener(consumer_key, consumer_secret, access_token, access_token_secret)
twitterListener.filter(track=['#HashTag'], threaded=True)

Where can this error be coming from? I am going mad trying to solve it... Everything should be in place...

But I keep getting this...

Stream encountered HTTP error: 403 Stream encountered HTTP error: 403 Stream encountered HTTP error: 403 Stream encountered HTTP error: 403

Thanks!