MQTT addon not transferring messages

Hi there,

I am completely new to home assistant, but I am familiar with mqtt. So far I did a fresh install on Raspberry Pi 3 model B+ (I also have issues with wifi configuration which does connect but the preparation screen then just hangs forever, but that’s another problem). After installation I followed the wizard to setup the basics, enabled advanced mode, and went to the supervisor in the menu and then to add ons and added mqtt addon. I am using this configuration:

logins: []
anonymous: true
customize:
  active: false
  folder: mosquitto
certfile: fullchain.pem
keyfile: privkey.pem
require_certificate: false

I started the server and tried to subscribe to it using my command line tools on my macbook:

Pims-Air:~ pim$ mqtt_sub -v -h 192.168.1.140 -p 1883 -t '#' -d
Client mosq-y5Haqq4Ugnx9wg1TcV sending CONNECT
Client mosq-y5Haqq4Ugnx9wg1TcV received CONNACK (0)
Client mosq-y5Haqq4Ugnx9wg1TcV sending SUBSCRIBE (Mid: 1, Topic: #, QoS: 0, Options: 0x00)
Client mosq-y5Haqq4Ugnx9wg1TcV received SUBACK
Subscribed (mid: 1): 128

So far so good, then to publish something over this mqtt server form a second terminal window:

Pims-Air:~ pim$ mqtt_pub -t 'test/testtopic' -m 'helloworld' -h 192.168.1.140 -d
Client mosq-dpclrJbiseW9E3DbTR sending CONNECT
Client mosq-dpclrJbiseW9E3DbTR received CONNACK (0)
Client mosq-dpclrJbiseW9E3DbTR sending PUBLISH (d0, q0, r0, m1, 'test/testtopic', ... (10 bytes))
Client mosq-dpclrJbiseW9E3DbTR sending DISCONNECT 

Seems to work fine… But nothing is coming out the other end on the first terminal window where I started the subscribe command.

Any ideas why the messages don’t seem to be transferred?

I can do the exact same thing with my other mqtt server (manually installed on another raspberry pi on my home network) and that works fine! So I am quite frustrated this isn’t working as expected. I really hope I am simply missing something obvious I am not seeing at the moment.

Thank you for any help!