Zigbee2MQTT web UI not accessible (502) on Home Assistant Green, despite successful startup

[vaclavIII]

Hi,
I'm trying to migrate from ZHA to Zigbee2MQTT on Home Assistant Green (running official OS with Supervisor).
Everything is working under the hood – coordinator starts correctly, MQTT is connected, and logs show "Zigbee2MQTT started!" and "HTTP server started on port 8080".
However, the web UI is never accessible. Clicking "Open web UI" results in 502 Bad Gateway.
Direct access via http://:8080 or other exposed ports also fails. I tried:
enabling and disabling frontend.port,
using frontend.external: true,
custom ports like 8099,
resetting add-on and reinstalling from official Zigbee2MQTT repo,
multiple full reboots.
Still no success. I suspect the Ingress or container port mapping is broken in my HA Green environment.
Any ideas, experience, or suggestions?
Thanks a lot in advance!
Vaclav

[vaclavIII]

The problem is (hopefully) solved, as follows:

1. the MQTT broker add-on installed cleanly (so far), without any further setup (user, login etc).
2. Zigbee2MQTT addon installed, after installation again 502. In the "serial" field of the GUI add-on, I then entered the exact port definition with the dongle coordinator (port: /dev/serial/by-id/usb-Nabu_Casa_SkyConnect_v1.0_cae4fXXXXXXXXXXXXXXX-if00-port0). I now receive the www page of the frontend. I'm a bit confused that the installation skipped the "onboarding page" phase compared to the instructions. My explanation is that the default settings are satisfactory, and that therefore filling in this page is not necessary.
   I haven't tried the other functionality yet - because I have to migrate all devices from ZHA>>Zigbee@MQTT, and I have to do that at once.
   I apologize for bothering the experts on this forum with my naive question...
   If anyone has a comment on the problem, further advice, I would be very happy. I'm a beginner, struggling a bit from Zigbee2MQTT - however I like the system, unlike ZHA it's not a black box.

[stale]

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