Varys fails to connect to servers behind a nginx proxy

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 Upgrade and Connection headers must be set explicitly, as in this example:

proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection "upgrade";
proxy_set_header Host $host;

Source: nginx.com