socket.io net::ERR_EMPTY_RESPONSE when connecting

[crupeng-1]

I am suddenly experiencing this error even though socket.io worked before. On the client side I checked if the connection was going to the correct place (it was), and if the redis-server was running (it was). Now I am out for ideas... How can I solve these empty responses?

Client

const {hostname, protocol} = location;
const socket = io(`${protocol}//${hostname}:3001`);

Server

const cluster = require("cluster");
const http = require("https");
const {Server} = require("socket.io");
const redisAdapter = require("socket.io-redis");
const numCPUs = require("os").cpus().length;
const {setupMaster, setupWorker} = require("@socket.io/sticky");
const origin = "http://localhost:3000";

if (cluster.isMaster) {
  const httpServer = http.createServer();

  setupMaster(httpServer, {
    loadBalancingMethod: "least-connection", 
  });
  httpServer.listen(3001);

  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  };

  cluster.on("exit", function () {
    cluster.fork();
  });
} else {
  const origin = "http://localhost:3000";
  const httpServer = http.createServer();
  const io = new Server(httpServer, {
    cors: {
      origin: [origin]
    }
  });

  io.adapter(redisAdapter({ 
    host: "localhost", 
    port: 6379 
  }));
  setupWorker(io);

  io.on("connection", function (socket) {
      //...
  };
};