I tried to deploy my meteor website with mupx-letsencrypt - but after doing mup deploy once, nothing works anymore

Question

So I used mupx-letsencrypt to get ssl for my website. Then I used mup deploy and ever since I get a 502 error when I go to the domain and a 503 error when I go to the IP address directly. Any hints appreciated.

This is my mup.json file

  {
  // Server authentication info
  "servers": [
    {
      "host": "123.123.123.123",
      "username": "root",
      "password": "password",
      // or pem file (ssh based authentication)
      // WARNING: Keys protected by a passphrase are not supported
      "pem": "/home/[myUser]/.ssh/id_rsa",
      // Also, for non-standard ssh port use this
      //"sshOptions": { "port" : 49154 },
      // server specific environment variables
      "env": {}
    }
  ],

  // Install MongoDB on the server. Does not destroy the local MongoDB on future setups
  "setupMongo": true,

  // Application name (no spaces).
  "appName": "[appName]",

  // Location of app (local directory). This can reference '~' as the users home directory.
  // i.e., "app": "~/Meteor/my-app",
  // This is the same as the line below.
  "app": ".",

  // Configure environment
  // ROOT_URL must be set to your correct domain (https or http)
  "env": {
    "PORT": 80,
    "ROOT_URL": "https://example.com" //also tried with http://
  },

  // Meteor Up checks if the app comes online just after the deployment.
  // Before mup checks that, it will wait for the number of seconds configured below.
  "deployCheckWaitTime": 15,

  // show a progress bar while uploading.
  // Make it false when you deploy using a CI box.
  "enableUploadProgressBar": false,

  // If letsEncrypt object exists - Meteor Up will generate Let's encrypt
  // certificates and automatically renew it every hour.
  "letsEncrypt": {
    "domain": "example.com",
    "email": "[email protected]"
  }
}

I have the feeling that I forgot some configuration that is not in this file as I tried all sorts of different combinations.