Admin UI/Dashboard javascript error

Hello all. Tinkering with dashboards and admin ui, only used appdaemon for automations before. Howver, I can’t get it to work. I get the following error in Firefox:

Source map error: Error: request failed with status 404
Resource URL: http://192.168.1.4:8456/javascript/socket.io.js
Source Map URL: socket.io.js.map

Firefox can’t establish a connection to the server at ws://192.168.1.4:8456/stream. reconnecting-websocket.min.js:1:1075
The connection to ws://192.168.1.4:8456/stream was interrupted while the page was loading. reconnecting-websocket.min.js:1:1075


All logs clean. Appdaemon 4.0.1 Config very simple:

appdaemon:
  plugins:
    HASS:
      type: hass
      ha_url: http://192.168.1.3:8123
      token: <token>
  time_zone: 'tz'
  latitude: nnn
  longitude: nnn
  elevation: 0
http:
  url: http://192.168.1.4:8456
#  transport: socketio
admin:

hadashboard:

I don’t even know where to start looking.
The ‘compiled/javascript’ folder only contains ‘application.js’ and ‘main_init.js’

Any ideas how to fix this or at least how to continue the search for the fault?

Edit: python 2.7.3 and 3.8.1 does not make any difference.