socket.io in useEffect produces Proxy Error (ECONNRESET) - node.js

Question

Summary
I've built a prototype for an app that is going to have some sort of chat functionality. As of right now the frontend React just pulls data via useEffect. To implement a more dynamic chat feeling I thought about using socket.io. When instantiating socket.io client-side as suggested (in useEffect) I am getting an ECONNRESET error.
Setup:
Backend: Node.js + express.js (listening on Port 5000)
Frontend: React
Frontend: Proxy for local development in package.json like this "proxy": "http://localhost:5000"
Problem:
When initializing my socket client-side like this:
const Flow = () => {
...
const fetchsocketData = () => {
const socket = io();
console.log("trying socket stuff");
socket.on("FromBackEnd", data => {console.log(data)});
};
useEffect(() => fetchsocketData(),[])
...
return (<div>Yolo</div>)
}
The proxying (as defined in the package.json) works nicely with e.g. Axios-calls, however, with socket.io, I get the following error on the server-side:
Proxy error: Could not proxy request
/socket.io/?EIO=3&transport=polling&t=N5v5GOe&sid=yugXlgWYsoqJRqcxAAAT
from localhost:3000 to http://localhost:5000. See
https://nodejs.org/api/errors.html#errors_common_system_errors for
more information (ECONNRESET).
And the following error on the client-side:
websocket.js:116 WebSocket connection to
'ws://localhost:3000/socket.io/?EIO=3&transport=websocket&sid=QKMDK2qmVGT3eud2AAAA'
failed: Error during WebSocket handshake: Unexpected response code:
400
This seems to be a temporal thing though, as the socket.io-connection is ultimately established and emits the test messages.
To make things a little weirder: If I move the socket instantiation into a user-triggered event:
const manuallyInstantiateSocket = () => {
const socket = io();
console.log("trying socket stuff");
socket.on("FromBackEnd", data => {console.log(data)});
}
And call this on a click of a button all works as expected. No error on the back-end side of things. The front-end error persists though.
What am I missing here?