Receiving a "[ERR] [object Event]" on page load

[jshwek]

I tried running this out of the box on an Ubuntu server.

I set it up on Nginx as a reverse proxy pointing to http://localhost:8000.

This worked fine on my Windows machine which displays:
Connected to PID 29124 [C:\Windows\System32\WindowsPowerShell\v1.0\powershell.exe] at COMPUTER-NAME (Node v17.6.0)
Main module: ../app.js

In the console, the error is (where APP_URL is my domain.):
WebSocket connection to 'wss://APP_URL/?id=0' failed:

It points to the _makeProtocolSocket function in index.html, specifically:
const socket = new WebSocket((window.location.protocol === 'http:' ? 'ws:' : 'wss:') + '//' + window.location.host + '/?id=' + id);