Probot: Don't receive any events from Webhook

[Schachte]

Bug Report

Reviving https://github.com/probot/probot/issues/636 as this issue is still happening for me.
Current Behavior

02:01:21.785Z ERROR probot: Not Found
  Error: Not Found
      at Request.callback (...../node_modules/smee-client/node_modules/superagent/lib/node/index.js:706:15)
      at IncomingMessage.parser (...../node_modules/smee-client/node_modules/superagent/lib/node/index.js:916:18)
      at IncomingMessage.emit (events.js:187:15)
      at IncomingMessage.EventEmitter.emit (domain.js:442:20)
      at endReadableNT (_stream_readable.js:1094:12)
      at process._tickCallback (internal/process/next_tick.js:63:19)

when trying to receive context of event from Webhook.

// Your code here

module.exports = app => {
  app.on('issues.opened', async context => {
    const params = context.issue({
      body: 'Hello World!'
    });
    await context.github.issues.createComment(params);
  });
};

Expected behavior/code

Should see that comment appears when new issue was opened.

Environment

Probot version(s): latest
Node/npm version: latest
OS: mojave

[welcome[bot]]

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[Schachte]

Hm, it actually does look like it fails when I have a react UI server running in parallel. Should we look into adding more explicit messages for port conflicts?