[Troubleshooting] Node.js app on App Engine Flex / Nginx connection reset by peer while proxying upgraded connection

We have a nodejs app using websocket that works perfectly fine in local development environment. After deploying the app to google app engine (flex environment), it works fine with a low number of requests but when the number of requests becomes important, the stream is closed and I get the following error : ``` "recv() failed (104: Connection reset by peer) while proxying upgraded connection, client: 142...212, server: , request: "GET /media HTTP/1.1", upstream: "http://..0.1:8080/media", host: "************.[login to view URL]". ``` I think it's not a code problem (since it works fine in local environment). My first thought is that it is due to the reverse proxy (nginx, which is used by default with Google). I have tried changing many parameters' values in the [login to view URL] file (put worker_connections to 16384 ...) but I still get the same error. The nginx config is given here [login to view URL]