error on establishing mqtt connection to IoT Core AwsCrtError: AWS_ERROR_MQTT_UNEXPECTED_HANGUP

I created a thing and downloaded the connectivity package to my raspberry pi. I used the Python flavor.
Running the default pubsub.py example works fine, that is no policy or certificate issue.

Now I wanted to create a python only solution that is not triggered by some cli tool. I used below code that I took from knowledge center but it gives me "AwsCrtError: AWS_ERROR_MQTT_UNEXPECTED_HANGUP error."

from awscrt import io, mqtt, auth, http
from awsiot import mqtt_connection_builder
import time as t
import json

# Define ENDPOINT, CLIENT_ID, PATH_TO_CERTIFICATE, PATH_TO_PRIVATE_KEY, PATH_TO_AMAZON_ROOT_CA_1, MESSAGE, TOPIC
ENDPOINT = "xxxx-ats.iot.eu-west-1.amazonaws.com"
CLIENT_ID = "mything"
PATH_TO_CERTIFICATE = "/home/pi/iotexample/connect_device_package/mything.cert.pem"
PATH_TO_PRIVATE_KEY = "/home/pi/iotexample/connect_device_package/mything.private.key"
PATH_TO_AMAZON_ROOT_CA_1 = "/home/pi/iotexample/connect_device_package/root-CA.crt"
TOPIC = "topic/#"

if __name__ == '__main__':
    # Spin up resources
    event_loop_group = io.EventLoopGroup(1)
    host_resolver = io.DefaultHostResolver(event_loop_group)
    client_bootstrap = io.ClientBootstrap(event_loop_group, host_resolver)
    mqtt_connection = mqtt_connection_builder.mtls_from_path(
                endpoint=ENDPOINT,
                cert_filepath=PATH_TO_CERTIFICATE,
                pri_key_filepath=PATH_TO_PRIVATE_KEY,
                client_bootstrap=client_bootstrap,
                ca_filepath=PATH_TO_AMAZON_ROOT_CA_1,
                client_id=CLIENT_ID,
                clean_session=False,
                keep_alive_secs=6
                )
    print("Connecting to {} with client ID '{}'...".format(
            ENDPOINT, CLIENT_ID))
    # Make the connect() call
    connect_future = mqtt_connection.connect()
    # Future.result() waits until a result is available
    connect_future.result()
    print("Connected!")

Anybody got an idea why?