Paho MQTT delivers "log: Caught exception in on_publish: 'int' object has no attribute 'topic'"

I wrote following python script:

import paho.mqtt.client as mqtt
import Adafruit_DHT
import time
import ssl
import logging

logging.basicConfig(level=logging.DEBUG, filename='/home/user/Dokumente/raspi.log', encoding='utf-8')

# Set up DHT22 sensor
dht_sensor = Adafruit_DHT.DHT22
dht_pin = 4  # GPIO pin number the sensor is connected to

username = 'simulation:mqttuser'
secret = 'ABCDEF'
host = '192.168.0.52'
port = 8883
clientID = 'raspi'
assetID = 'A1B2C3D4'


def on_connect(client, userdata, flags, rc):
    print("Connected with result code "+str(rc))
    logging.info("Connected with result code "+str(rc))


def on_publish(client, userdata, msg):
    print(msg.topic+" "+str(msg.payload))
    logging.info(msg.topic+" "+str(msg.payload))


def on_disconnect(client, userdata, rc):
    if rc != 0:
        print("Unexpected disconnection.")
        logging.info(f"Unexpected disconnection. Reason: {mqtt.connack_string(rc)}")


def on_message(client, userdata, msg):
    print("message received ", str(msg.payload.decode("utf-8")))
    print("message topic=", msg.topic)
    print("message qos=", msg.qos)
    print("message retain flag=", msg.retain)


def on_log(client, userdata, level, buf):
    print("log: ", buf)
    # logging.info("log: ", buf)


mqtt_client = mqtt.Client(clientID)  # create new instance
mqtt_client.username_pw_set(username, password=secret)
context = ssl.SSLContext(ssl.PROTOCOL_TLSv1_2)
mqtt_client.tls_set_context(context)

# Callback functions
mqtt_client.on_connect = on_connect  # attach function to callback
mqtt_client.on_disconnect = on_disconnect
mqtt_client.on_publish = on_publish
mqtt_client.on_message = on_message
mqtt_client.on_log = on_log

mqtt_client.connect(host, port=port, keepalive=930)  # connect to broker; keepalive must be higher than time.sleep (see below)
mqtt_client.loop_start()  # start loop to process callbacks
mqtt_client.subscribe(f"simulation/{clientID}/writeattributevalue")


# Continuously read and send temperature values
while True:
    humidity, temperature = Adafruit_DHT.read_retry(dht_sensor, dht_pin)
    if humidity is not None and temperature is not None:
        result = mqtt_client.publish(f"simulation/{clientID}/writeattributevalue/temperature/{assetID}", round(temperature, 1))
        mqtt_client.publish(f"simulation/{clientID}/writeattributevalue/humidity/{assetID}", humidity)
        if result.rc == mqtt.MQTT_ERR_SUCCESS:
            logging.info("Message published successfully")
        else:
            logging.warning("Failed to publish message: " + mqtt.error_string(result.rc))
    else:
        print("Failed to read temperature sensor data")
    time.sleep(10)

# Disconnect MQTT client
mqtt_client.disconnect()
mqtt_client.loop_stop()

When executing it, following exception occurs:

log:  Sending CONNECT (u1, p1, wr0, wq0, wf0, c1, k930) client_id=b'raspi_musterstadt'
log:  Sending SUBSCRIBE (d0, m1) [(b'simulation/raspi_musterstadt/writeattributevalue', 0)]
log:  Sending PUBLISH (d0, q0, r0, m2), 'b'simulation/raspi_musterstadt/writeattributevalue/temperatur/7O1Rq2zsU4ByAgBUcFRq26'', ... (4 bytes)
log:  Received CONNACK (0, 0)
Connected with result code 0
log:  Received SUBACK
log:  Sending PUBLISH (d0, q0, r0, m3), 'b'simulation/raspi_musterstadt/writeattributevalue/luftfeuchtigkeit/7O1Rq2zsU4ByAgBUcFRq26'', ... (18 bytes)
log:  Caught exception in on_publish: 'int' object has no attribute 'topic'
log:  Caught exception in on_publish: 'int' object has no attribute 'topic'

Does someone know what the problem is? The ‘msg’ parameter of the ‘on_publish’ method has appearantly no topic or payload and I have no clue why. If I type in the print-statement print(msg) it delivers an integer value, counting up from 3 (next loop: 4, …). I appreciate any help!