Heroku crash over and over by POST request, multer or nodemailer H=18

My deployment on Heroku keep crashing on the POST request when I send the mulitpart form. I can't see in the logs if it's on the upload (multer) function, save (mongoose) function or sendMail (nodemailer) function. The only thing I see in the logs is a H18 error: Internal Server.
Router.js

const express = require("express");
const mongoose = require("mongoose");
const router = express.Router();
const multer = require("multer");
const path = require("path");
const File = require("../models/Files");
const mail = require("../handlers/mailer");

// Set storage engine
const storage = multer.memoryStorage();

// Init upload
const upload = multer({
  storage: storage
}).single("file");

router.get("/", (req, res) => {
  res.render("index");
});

router.post("/send", async (req, res, next) => {

  await upload(req, res, async err => {
    if (err) {
      console.log("error by uploading file:", err);
    } else {
      console.log(`File is uploaded to the memoryStorage: ${req.file.originalname} `);
    }

    // Create a model to save in the database
    const fileUpload = new File({
      fromEmail: "<dk@bigbrother.nl>",
      fromName: '"Dennis Klarenbeek 👻"',
      email: req.body.email,
      subject: req.body.subject,
      msg: req.body.msg,
      filename: req.file.originalname
    });

    await fileUpload.save((err, file, rows) => {
      if (err) {
        console.log("error on saving in the db");
      } else {
        console.log(`database item has been created: ${file.filename}`);
      }
    });

    // Mail the uploaded attachment
    await mail.send({
      fromEmail: "dennis.klarenbeek@icloud.com",
      fromName: '"Dennis Klarenbeek 👻"',
      toEmail: req.body.email,
      toName: req.body.name,
      subject: req.body.subject,
      msg: req.body.msg,
      template: "attachment",
      attachments: [
        {
          filename: req.file.filename,
          contentType: req.file.mimetype,
          content: req.file.buffer
        }
      ]
    });
  });

  res.redirect("/");
});

module.exports = router;

Logs

2018-07-12T15:29:46.104415+00:00 heroku[router]: at=info method=GET path="/css/style.css" host=stormy-ocean-50061.herokuapp.com request_id=57113d1c-9730-40ca-9f41-0d5111854175 fwd="87.251.40.140" dyno=web.1 connect=1ms service=10ms status=304 bytes=237 protocol=https
2018-07-12T15:29:46.103429+00:00 heroku[router]: at=info method=GET path="/css/normalize.css" host=stormy-ocean-50061.herokuapp.com request_id=44a0f90b-1973-4daf-9f40-1e5e5398b9e4 fwd="87.251.40.140" dyno=web.1 connect=1ms service=7ms status=304 bytes=238 protocol=https
2018-07-12T15:29:46.487118+00:00 app[web.1]: [0mGET /webfonts/fa-light-300.woff2 [36m304 [0m0.353 ms - -[0m
2018-07-12T15:29:46.489183+00:00 heroku[router]: at=info method=GET path="/webfonts/fa-light-300.woff2" host=stormy-ocean-50061.herokuapp.com request_id=885eae11-3e4c-4efa-9b60-b3950d9f256d fwd="87.251.40.140" dyno=web.1 connect=1ms service=2ms status=304 bytes=239 protocol=https
2018-07-12T15:29:56.919861+00:00 app[web.1]: [0mPOST /send [36m302 [0m9.041 ms - 46[0m
2018-07-12T15:29:57.100559+00:00 heroku[router]: sock=backend at=error code=H18 desc="Server Request Interrupted" method=POST path="/send" host=stormy-ocean-50061.herokuapp.com request_id=aaafb074-b538-4983-bef1-fa1abf1f2413 fwd="87.251.40.140" dyno=web.1 connect=1ms service=191ms status=503 bytes=234 protocol=https

Does somebody know what this could be?


from Heroku crash over and over by POST request, multer or nodemailer H=18