Problems with fetching data on Express server

I created NodeJS server with Express.js and now I’m trying to make a request for it by fetch.

fetch("http://localhost:3001/api", {
    method: "POST",
    headers: {
      "Content-Type": "application/json",
    },
    body: JSON.stringify(formData),
  })
    .then((response) => response.json())
    .then((data) => {
      if (data.user) {
        alert(`Welcome, ${data.user.name}!`);
      } else {
        alert("User not found!");
      }
    })
    .catch((error) => {
      alert("Error fetching data:" + error);
    });

This is server part:

const express = require('express');
const cors = require('cors');
const bodyParser = require('body-parser');

const app = express();
app.use(cors());
app.use(bodyParser.json());

const users = {
  AG0n1: {
    name: "AG0n1",
    status: "admin",
    email: "[email protected]",
    password: "111",
    phoneNumber: "+375293883088",
  },
  Teacher: {
    name: "Teacher",
    status: "admin",
    email: "admin",
    password: "test",
    phoneNumber: "+375293883088",
  },
  User: {
    name: "User",
    status: "user",
    test: "test",
  },
};

app.post('/api', (req, res) => {
  console.log("Received a POST request");
  const { email, password } = req.body;
  const user = Object.values(users).find((u) => u.email === email && u.password === password);
  if (user) {
    console.log(user)
    res.json({ user });
  } else {
    res.status(401).json({ error: 'Invalid credentials' });
  }
});


app.get('/api', (req, res) => {
  res.json(users);
});

But sometimes at random moment I get the following error:
Error fetching data:TypeError: Failed to fetch

At first glance, there seems to be no pattern to this error. Sometimes I get the expected result and sometimes this error.
Can you help me to solve this problem?

ChatGPT said that my code is normal and that I used middleware ‘cors()’. He didnt help me so i decided to ask this question here.