Vaadin @Push error on Load Balancer


I have problem with Vaadin @Push when we deploy the app with Load Balancer.
Basically, we have 2 app instances ( 2 Tomcats in a virtual Linux machine) and the LB in other virtual Linux machine (fig. 1).
when we access to LB’s URL, Vaad @Push show a error as bellow (fig. 2):

This is possible Vaadin @Push issue or LB config ?

I would vote for load balancer config issue. In Flow Push uses by default WebSocket.XHR, which means that communication is websocket in other direction and http in the other. This means that you need to configure the load balancer to handle both http connections and websocket connections. It seems that websocket part of the configuration is missing.

yes, it is high possible for the LB configuration problem. Actually we also configured websocket for LB and if we turn off 1 app instance, keep only the LB and 1 app instance, then everything is fine.

The issue only happens if LB runs with 2 app instances.

i think this is issue with Vaadin push implementation, not load balancer configuration.
Submitted a issue:

now receive new error message in chrome:
Failed to load resource: net::ERR_HTTP2_PROTOCOL_ERROR

note: i have a custom js loaded to client and it can access to our LB via websocket as normal.
so strong believe this is a VaadinPush.js issue