Mosquitto broker is working, but: Failed to connect to MQTT server due to exception: [Errno 113] Host is unreachable

I am trying to connect HomeAssistant to the Mosquito Broker, but I’m out of ideas…

I’m using the Mosquitto broker from the Add-on store in Version 5.1 with hassOS on a raspi4 with all the latest updates (see below for system info).
The broker is up and running. I can see it in the logs of the broker, I can connect to it with MQTT Explorer (<internal_ip> below in the logs) and owntracks on my mobile phone is successfully publishing messages (my phone publishes them, I see them in MQTT Explorer and the broker logs; <external_id> in the logs, connected via a dyndns).
But the home assistant logs tell me:

 Log Details (ERROR)

Logger: homeassistant.components.mqtt
Source: components/mqtt/__init__.py:748
Integration: MQTT (documentation, issues)
First occurred: 9:50:40 PM (1 occurrences) 
Last logged: 9:50:40 PM

Failed to connect to MQTT server due to exception: [Errno 113] Host is unreachable

Any ideas? I am grateful for any help! Thanks


These are the logs of the broker:

[21:49:01] INFO: Setup mosquitto configuration
[21:49:01] INFO: Found local users inside config
[21:49:01] INFO: Initialize Hass.io Add-on services
[21:49:01] INFO: Initialize Home Assistant discovery
[21:49:01] INFO: Start Mosquitto daemon
1600112941: mosquitto version 1.6.3 starting
1600112941: Config loaded from /etc/mosquitto.conf.
1600112941: Loading plugin: /usr/share/mosquitto/auth-plug.so
1600112941:  ├── Username/password checking enabled.
1600112941:  ├── TLS-PSK checking enabled.
1600112941:  └── Extended authentication not enabled.
1600112941: |-- *** auth-plug: startup
1600112941: Opening ipv4 listen socket on port 1883.
1600112941: Opening ipv6 listen socket on port 1883.
1600112941: Opening websockets listen socket on port 1884.
1600112941: Opening ipv4 listen socket on port 8883.
1600112941: Opening ipv6 listen socket on port 8883.
1600112941: Opening websockets listen socket on port 8884.
1600112941: Warning: Mosquitto should not be run as root/administrator.
1600112947: New connection from <internal_ip> on port 1883.
[INFO] found mosquittoUser on local database
1600112947: New client connected from <internal_ip> as mqtt-explorer-4a6ec9b6 (p2, c1, k60, u'mosquittoUser').
1600112989: New connection from <external_ip> on port 1883.
1600112989: New client connected from <external_ip> as mosquittoUserowntrackiphone6s (p2, c0, k60, u'mosquittoUser').
1600112993: Client mosquittoUserowntrackiphone6s disconnected.
1600112993: New connection from <external_ip> on port 1883.
1600112993: New client connected from <external_ip> as mosquittoUserowntrackiphone6s (p2, c0, k60, u'mosquittoUser').
1600112997: Client mosquittoUserowntrackiphone6s disconnected.
1600113277: New connection from <external_ip> on port 1883.

This is my broker config:

logins:
  - username: mosquittoUser
    password: <PASSWORD>
anonymous: false
customize:
  active: false
  folder: mosquitto
certfile: fullchain.pem
keyfile: privkey.pem
require_certificate: false
client_id: home_assistant

This is my system info:

arch 	armv7l
chassis 	embedded
dev 	false
docker 	true
docker_version 	19.03.11
hassio 	true
host_os 	HassOS 4.13
installation_type 	Home Assistant OS
os_name 	Linux
os_version 	4.19.127-v7l
python_version 	3.8.3
supervisor 	243
timezone 	Europe/Berlin
version 	0.114.4
virtualenv 	false
Lovelace
dashboards 	1
mode 	storage
resources 	8
views 	4