Cross-Origin Request Blocked on Ubuntu Webserver, regardless of Access-Control-Allow-Origin Headers

the following problem. I have an Ubuntu web server running a React app. In particular, there are two files in the server file that process the requests made by the website. For some reason, however, it is not possible to make a request to the server as a client.

Server.js:

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

const app = express()
app.use(cors({
    origin: ['*']
}))
app.use(bodyParser.json());

const db = mysql.createConnection({
    host: '192.168.178.70',
    user: 'webserver',
    password: '123',
    database: 'lanparty',
    port: '3306'
})


app.get('/Ergebnisse', (req, res) => {
    console.log("gotRequest")

    db.query("SELECT * FROM lanparty.match;", (err, data) => {
        if (err) {
            console.log(err)
            res.json(err)
        } else {
            res.json(data)
        }  
    })
})


app.listen(30100, () => {
    console.log("Listening....")
})

Results.js:

import { useEffect } from "react"
import axios from "axios";

export default function Results() {

  useEffect(() => {
    console.log("fetching...")
    axios.get('http://localhost:30100/Results')
      .then(response => response.json())
      .then(data => {
        console.log("data")
        console.log(data)
      })
      .catch((error) => {
        console.error("Error:")
        console.log(error)
      });

  }, [])

  return (
    <div className="pageDiv">

      <h2>Results</h2>

    </div>
  );
}

If I´m making the request directly in the browser ( http://192.168.178.70:30100/Results ) everything works fine, so the server.js should work, but the Request gets blocked if the webserver is the origin.The exact error looks like this:

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource at http://localhost:30100/Results. (Reason: CORS request did not succeed). Status code: (null).

The request in the dev tools looks like this:
description of the error

I tried literally every way of adding the Access-Control-Allow-Origin Header:

• The way you see in my code right now
• The way you see in my code right now, but with every possibility to represent localhost (‘localhost’, ‘127.0.0.1’, the ip address of the webserver)
• In my app.get, in the form of res.setHeader("Access-Control-Allow-Origin", "localhost");
  again I tried "*" and "127.0.0.1"

but non of these worked for me.

Thank you in advance for your time and help 🙂