React Express Built app not working in production mode - reactjs

Question

I have a Create React App working well in development mode both with
yarn start
and
yarn build > serve -s build
But when I upload my build folder to my distant server (on Planet Hoster), I get the following errors :
GEThttps://www.mysite.fr/static/js/main.f79317f8.js
[HTTP/2 404 Not Found 273ms]
GEThttps://www.mysite.fr/static/css/main.43a07738.css
[HTTP/2 404 Not Found 272ms]
What's happening here ? I've been working days and days on it and I still can't find something... Any help will be greatful !
My code to launch my express server :
const express = require("express");
const path = require("path");
const app = express();
app.use(express.static(path.join(__dirname, "build")));
app.get("/", function (req, res) {
res.sendFile(path.join(__dirname, "build", "index.html"));
});
[EDIT] - Here is my package.json file if it can helps :
{
"name": "frontend",
"version": "0.1.0",
"private": true,
"dependencies": {
"#testing-library/jest-dom": "^5.16.1",
"#testing-library/react": "^12.1.2",
"#testing-library/user-event": "^13.5.0",
"axios": "^0.25.0",
"cors": "^2.8.5",
"express": "^4.17.2",
"react": "^17.0.2",
"react-dom": "^17.0.2",
"react-router": "^6.2.1",
"react-router-dom": "^6.2.1",
"react-router-hash-link": "^2.4.3",
"react-scripts": "5.0.0",
"web-vitals": "^2.1.4",
"webpack": "^5.67.0"
},
"scripts": {
"start": "react-scripts start",
"build": "react-scripts build",
"test": "react-scripts test",
"eject": "react-scripts eject"
},
"eslintConfig": {
"extends": [
"react-app",
"react-app/jest"
]
},
"browserslist": {
"production": [
">0.2%",
"not dead",
"not op_mini all"
],
"development": [
"last 1 chrome version",
"last 1 firefox version",
"last 1 safari version"
]
}
}

Answer 1

I am coming back to it lately but I finally found the solution.
To achieve it, I ran locally node app.js wich is the file that launch my frontend service. I then found in my logs that my script was trying to "modify the headers of my site after they were sent". I fix it and now it works.
My code was probably not that clean.
Still, thank you for your help !