I created such a mini program

from flask import Flask, request, jsonify
import serial

app = Flask(__name__)

ser = serial.Serial('COM3', 9600, timeout=1)

@app.route('/send', methods=['POST'])
def send():
    data = request.get_json().get('data', '')
    
    if data:
        write_to_port(ser, data)
        response = read_full_response(ser)
        return jsonify({"message": response}), 200
    else:
        return jsonify({"error": "No data provided"}), 400

@app.route('/read', methods=['GET'])
def read():
    reading = read_from_port(ser)
    return jsonify({"message": reading}), 200

def read_from_port(ser):
    reading = ser.readline().decode('utf-8').strip()
    print('Read:', reading)
    return reading

def write_to_port(ser, data):
    ser.write(data.encode())
    print(f"Written: {data}")

def read_full_response(ser):
    response = b""

    while True:
        if ser.in_waiting:
            chunk = ser.read(ser.in_waiting)
            response += chunk
            if b"x00" in chunk:
                break

    return response.decode('utf-8').strip()

if __name__ == '__main__':
    app.run(debug=True, host='0.0.0.0', port=5000)

On one computer it works fine, on the other I get an error

Traceback (most recent call last):
  File "C:terminalmain.py", line 8, in <module>
    ser = serial.Serial('COM3', 9600, timeout=1)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "C:UsersAdminAppDataLocalProgramsPythonPython312Libsite-packagesserialserialwin32.py", line 33, in __init__
    super(Serial, self).__init__(*args, **kwargs)
  File "C:UsersAdminAppDataLocalProgramsPythonPython312Libsite-packagesserialserialutil.py", line 244, in __init__
    self.open()
  File "C:UsersAdminAppDataLocalProgramsPythonPython312Libsite-packagesserialserialwin32.py", line 64, in open
    raise SerialException("could not open port {!r}: {!r}".format(self.portstr, ctypes.WinError()))
serial.serialutil.SerialException: could not open port 'COM3': PermissionError(13, 'Access is denied.', None, 5)

As far as I could check, there are no other programs.

I turned off and on this device in the Manager – does not help.

I run the cmd command as an administrator. Maybe someone else has ideas?