Unable to install ACE Jira app from Heroku

[John_Doe]

I've got my ACE app set up using the instructions at the ACE BitBucket site, and I've been manually deploying my app to development instances using ngrok tunnels as usual. The app works fine, all pages show properly and the application itself does what it's supposed to (in development mode)

Now, I'm trying to use Heroku for my production deployments, and the app builds and deploys just fine, with no errors showing in `heroku logs --tail`. I can clearly see the `atlassian-connect.json` descriptor being deployed to the appropriate URL on Heroku.

However, when I try to use URL to the deployed descriptor to install my app in one of my test Jira instances, I get the following error:

The app host returned HTTP response code 503 when we tried to contact it during installation. Please try again later or contact the app vendor.

These are the logs I get from Heroku, with no obvious errors being thrown anywhere:

2020-04-03T04:51:11.256336+00:00 heroku[router]: at=info method=GET path="/atlassian-connect.json" host=example-app.herokuapp.com request_id=4676badf-c37f-4440-b017-1b1ec5f00023 fwd="18.234.32.227" dyno=web.1 connect=1ms service=4ms status=200 bytes=1354 protocol=https

2020-04-03T04:51:11.255764+00:00 app[web.1]: ::ffff:10.5.179.3 - - [03/Apr/2020:04:51:11 +0000] "GET /atlassian-connect.json HTTP/1.1" 200 905 "-" "Apache-HttpClient/4.5.12 (Java/1.8.0_242)"

Lastly, here's my atlassian-connect.json file:

{
  "key": "example-app",
  "name": "Example App",
  "baseUrl": "{{localBaseUrl}}",
  "authentication": {
    "type": "jwt"
  },
  "lifecycle": {
    "installed": "installed"
  },
  "scopes": [
    "READ"
  ],
  "modules": {
    "jiraProjectPages": [
      {
        "key": "example-app-page-jira",
        "location": "system.top.navigation.bar",
        "name": {
          "value": "Example app"
        },
        "url": "/example-app",
        "iconUrl": "{{localBaseUrl}}/images/example.svg",
        "conditions": [{
          "condition": "user_is_logged_in"
        }]
      },
      {
        "key": "example-app-2-page-jira",
        "location": "system.top.navigation.bar",
        "name": {
          "value": "Example app 2"
        },
        "url": "/example-app-2",
        "iconUrl": "{{localBaseUrl}}/images/example2.svg",
        "conditions": [{
          "condition": "user_is_logged_in"
        }]
      }
    ]
  }
}

Here's what I've tried so far:

• Defining a route for `/installed` in `index.js`, giving the status code 200, like so:

app.get('/installed', (req, res) => {
res.sendStatus(200);
});

• Defining an additional route for `/installed` for POST requests
• Updating the `pg` and `sequelize` packages
• Installing the `jugglingdb` and `jugglingdb-postgres` packages as per this AC question 
• Uploading the descriptor as a private app to the Atlassian Marketplace and installing from there
• Making sure development mode was enabled on my Jira instance

Nothing seems to work, and the 503 error still persists; does anyone have any idea what's causing this error?