Heroku hosted JS and EJS app crashing when users are logging in

Question

I am new to programming so I don't know why this happens. Users are reporting app crashes when logging in. The app is hosted on the Heroku platform with MongoDB. Can anyone tell me what the problem might be? P.S. I don't know if there is any other way to show the code, so I'll just paste it below. The user and password for the MongoDB connection string are changed for obvious reasons.

//jshint esversion:6
const express = require("express");
const bodyParser = require("body-parser");
const ejs = require("ejs");
const MongoClient = require("mongodb").MongoClient;

const app = express();

const uri =
  "mongodb+srv://user:password@cluster0.4wwjt.mongodb.net/anketa?retryWrites=true&w=majority";

const client = new MongoClient(uri, {
  useNewUrlParser: true,
  useUnifiedTopology: true,
});

app.set("view engine", "ejs");
app.use(
  bodyParser.urlencoded({
    extended: true,
  })
);
app.use(express.static("public"));

app.get("/", (req, res) => {
  res.render("login");
});

app.get("/link", (req, res) => {
  res.render("link");
});

app.get("/confirm", (req, res) => {
  res.render("confirm");
});

app.get("/noMoreLinks", (req, res) => {
  res.render("noMoreLinks");
});

app.post("/link", (req, res) => {
  const username = req.body.username;
  const password = req.body.password;

  client.connect((err) => {
    const users = client.db("anketa").collection("users");
    const links = client.db("anketa").collection("links");

    users.findOne({ user: username }, (err, foundUser) => {
      if (err) {
        console.log(err);
      } else {
        if (foundUser && foundUser.pass === password) {
          const unvisitedLinksLength = foundUser.unvisitedLinks.length;
          const currentTime = Date.now();
          if (
            foundUser.timeFinished &&
            currentTime - foundUser.timeFinished < 18 * 60 * 60 * 1000
          ) {
            res.render("comeTomorrow", {
              headline: "Probajte opet za nekoliko sati",
            });
            return;
          }
          if (unvisitedLinksLength === 20) {
            links.findOne({ id: 1 }, (err, foundLink) => {
              if (err) {
                console.log(err);
              } else {
                res.render("link", {
                  link: foundLink.path,
                });
                users.update(
                  { user: foundUser.user },
                  { $set: { currentLink: foundLink.id } }
                );
              }
            });
            return;
          }
          if (unvisitedLinksLength < 20 && unvisitedLinksLength > 0) {
            let randomNumber = Math.floor(Math.random() * unvisitedLinksLength);
            links.findOne(
              { id: foundUser.unvisitedLinks[randomNumber] },
              (err, foundLink) => {
                res.render("link", {
                  link: foundLink.path,
                });
                users.update(
                  { user: foundUser.user },
                  { $set: { currentLink: foundLink.id } }
                );
              }
            );
            return;
          }
          if (unvisitedLinksLength === 0 && foundUser.currentLink < 21) {
            links.findOne({ id: foundUser.currentLink }, (err, foundLink) => {
              res.render("link", {
                link: foundLink.path,
              });
            });
            return;
          }
          if (foundUser.currentLink === 0 && unvisitedLinksLength === 0) {
            res.render("noMoreLinks");
            return;
          }
        }
      }
    });
  });
});

app.post("/confirm", (req, res) => {
  const username = req.body.username;
  client.connect((err) => {
    const users = client.db("anketa").collection("users");

    users.findOne({ user: username }, (err, foundUser) => {
      const currentLinkId = foundUser.currentLink;
      const unvisitedLinks = foundUser.unvisitedLinks;
      const newUnvisitedLinks = unvisitedLinks.filter(
        (linkId) => linkId !== currentLinkId
      );
      users.update(
        { user: foundUser.user },
        {
          $set: { unvisitedLinks: newUnvisitedLinks, timeFinished: Date.now() },
        }
      );
      if (unvisitedLinks.length === 0) {
        users.update({ user: foundUser.user }, { $set: { currentLink: 0 } });
        return;
      }
    });
  });
  res.render("login");
});

let port = process.env.PORT;
if (port == null || port == "") {
  port = 3000;
}

app.listen(port, () => {
  console.log("server started successfully");
});