Flask-SocketIO WebSocket transport is not available - flask

Question

I've got a Flask server with Flask-SocketIO to do instant reloads on some pages. Now I'm getting this message in the console:
The WebSocket transport is not available, you must install a WebSocket server that is compatible with your async mode to enable it. See the documentation for details.
Like discribed here https://github.com/miguelgrinberg/Flask-SocketIO/issues/647 I tried to install gevent or eventlet, but neither one changed anything.
The code on the Client is like this:
import { io } from "https://cdn.socket.io/4.4.1/socket.io.esm.min.js";
var socket = io();
socket.emit(event,data,(response) => {response_func(response);})
On the Server:
#socketio.on(event)
def functionOnEvent(data):
functions(data)
return data
Both code samples are of course simplified, but there is no async call of any sort.
Update:
After installing eventlet again and manually setting the async_mode the message disappeared. But now I get the following error from eventlet:
ERROR:Error on request:
Traceback (most recent call last):
File "C:\...\venv\lib\site-packages\werkzeug\serving.py", line 319, in run_wsgi
execute(self.server.app)
File "C:\...\venv\lib\site-packages\werkzeug\serving.py", line 306, in execute
application_iter = app(environ, start_response)
File "C:\...\venv\lib\site-packages\flask\app.py", line 2095, in __call__
return self.wsgi_app(environ, start_response)
File "C:\...\venv\lib\site-packages\flask_socketio\__init__.py", line 43, in __call__
return super(_SocketIOMiddleware, self).__call__(environ,
File "C:\...\venv\lib\site-packages\engineio\middleware.py", line 63, in __call__
return self.engineio_app.handle_request(environ, start_response)
File "C:\...\venv\lib\site-packages\socketio\server.py", line 597, in handle_request
return self.eio.handle_request(environ, start_response)
File "C:\...\venv\lib\site-packages\engineio\server.py", line 411, in handle_request
packets = socket.handle_get_request(
File "C:\...\venv\lib\site-packages\engineio\socket.py", line 103, in handle_get_request
return getattr(self, '_upgrade_' + transport)(environ,
File "C:\...\venv\lib\site-packages\engineio\socket.py", line 158, in _upgrade_websocket
return ws(environ, start_response)
File "C:\...\venv\lib\site-packages\engineio\async_drivers\eventlet.py", line 16, in __call__
raise RuntimeError('You need to use the eventlet server. '
RuntimeError: You need to use the eventlet server. See the Deployment section of the documentation for more information.
As far as I unterstand and show in this example https://github.com/miguelgrinberg/Flask-SocketIO-Chat I thought when running the app like socketio.run(app), it should automatically run an eventlet server?