Varys communicates with your Plex server using the websocket protocol. Please make sure you configured your proxy server correctly to be able to handle websockets.
NGINX supports WebSocket by allowing a tunnel to be set up between a client and a backend server. For NGINX to send the Upgrade request from the client to the backend server, the
Connectionheaders must be set explicitly, as in this example:
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";
proxy_set_header Host $host;