EventStream empty though events come?

Question

JavaScript

Tested in different ways

// const event = new EventSource(url);
        // const event = new EventSource(url, {withCredentials: true});
        let mercureToken = 'Bearer ' + Cookies.get("mercureAuthorization");
         const event = new EventSourcePolyfill(
            url,
            {
                heartbeatTimeout: 3600*1000,
                headers: {
                  'Authorization': mercureToken,
              },
            },
        );

Any of the approaches, the tab with events is empty. It is very difficult to debug what events arrive.
Used Mercure + caddy server (Debian 10 stretch), symfony (Debian 10 stretch) 2 servers with CORS requests .
I'm sure it's because of some kind of js.

SO answer didn't help.