Zigbee2mqtt add-on: 502: Bad Gateway

Configuration
1

Hello,

Trying to get the add-on zigbee2mqtt to work in Home Assistant, but get the error “502: Bad Gateway” when trying to open Web UI. Below is the log file.
Any input is welcome.

s6-rc: info: service z2m: starting
s6-rc: info: service socat: starting
s6-rc: info: service s6rc-oneshot-runner: starting
s6-rc: info: service z2m successfully started
s6-rc: info: service socat successfully started
s6-rc: info: service s6rc-oneshot-runner successfully started
s6-rc: info: service fix-attrs: starting
s6-rc: info: service fix-attrs successfully started
s6-rc: info: service legacy-cont-init: starting
s6-rc: info: service legacy-cont-init successfully started
s6-rc: info: service legacy-services: starting
s6-rc: info: service legacy-services successfully started
[11:17:36] INFO: Socat not enabled
[11:17:37] INFO: Handing over control to Zigbee2MQTT Core ...
Zigbee2MQTT:info  2022-09-14 11:17:42: Logging to console and directory: '/config/zigbee2mqtt/log/2022-09-14.11-17-42' filename: log.txt
Zigbee2MQTT:info  2022-09-14 11:17:42: Starting Zigbee2MQTT version 1.27.2 (commit #unknown)
Zigbee2MQTT:info  2022-09-14 11:17:42: Starting zigbee-herdsman (0.14.53)
Zigbee2MQTT:info  2022-09-14 11:17:43: zigbee-herdsman started (resumed)
Zigbee2MQTT:info  2022-09-14 11:17:43: Coordinator firmware version: '{"meta":{"maintrel":1,"majorrel":2,"minorrel":7,"product":1,"revision":20210708,"transportrev":2},"type":"zStack3x0"}'
Zigbee2MQTT:info  2022-09-14 11:17:43: Currently 0 devices are joined:
Zigbee2MQTT:info  2022-09-14 11:17:43: Zigbee: disabling joining new devices.
Zigbee2MQTT:info  2022-09-14 11:17:43: Connecting to MQTT server at diskstation:1883

And the configuration file:

homeassistant: true
mqtt:
  server: mqtt://diskstation:1883
serial:
  port: /dev/ttyUSB0
frontend:
  port: 8099
advanced:
  homeassistant_legacy_entity_attributes: false
  legacy_api: false
  legacy_availability_payload: false
device_options:
  legacy: false
2

Found the issue, I see from the log that it never succeed in connecting to the mqtt broker. Turned out to be the format of server definition:
image