fasthttp: Error: "too many open files" for

I have one very unpleasant issue with the fasthttp : There is a “highly loaded” server that stops processing requests after 5-10 minutes (about 5000 requests per minute) with error: Temporary error when accepting new connections: accept tcp4 IP:PORT: accept: too many open files

Tried 2 ways to start the (with standard ListenAndServe and with custom Server options (disabled Keepalive, custom timeouts and closing connection on handers) but isn’t works.

func HTTPServer() {
	listenAddress := strings.Join([]string{os.Getenv("SERVER_IP"), os.Getenv("SERVER_PORT")}, ":")
	log.Printf("Starting server on %q", listenAddress)
	go func() {
		if err := server.ListenAndServe(listenAddress, CustomModelHandler); err != nil {
			log.Fatalf("error in ListenAndServe: %s", err)
		}
	}()
}


func CustomModelHandler(ctx *server.RequestCtx) {
	var customModel models.CustomModel
	err := json.Unmarshal(ctx.PostBody(), &customModel)
	if err != nil {
		ctx.Error(err.Error(), server.StatusBadRequest)
		return
	}
        // ....
	// some actions with customModel
        // ....
	if updated {
		ctx.SetStatusCode(server.StatusOK)
	} else {
		ctx.SetStatusCode(server.StatusCreated)
	}

	defer ctx.Request.Reset()
	defer ctx.Request.SetConnectionClose()
}
`

Any ideas how to fix it?