ESP32 DevKit WIFI+BLE WebServer POST problem

Sorry, I can't post that code. I have deleted it. I believe that I didn't use .end(). So it can be the reason.
Though there is a part of new code that works fine now:

// On POST Request Handler
function onPOST(req, res) {
  let data = '';
  req.on('data', d => data += d);
  req.on('end', () => {
    try {
      let obj = JSON.parse(data);
      mod = obj.mode;
      res.writeHead(200, { 'Content-Type': 'text/plain' });
      res.end('Mode Set');
    } catch (err) {
      res.writeHead(200, { 'Content-Type': 'text/plain' });
      res.end(`Error: ${err}`);
    }
  });
}