The error message is misleading. It happens if a client try to contact the proxy entrypoint with the HTTP protocol without requesting an upgrade to Websocket : only websocket requests can be tunneled to a websockend endpoint, thus the proxy complains.
Your web application should have a subset of paths which use Websockets and all other paths with plain HTTP(S). The correct config often boils down to : Proxy /ws ws://host:port/ Proxy / http://host:port/ And DON'T try to test by requesting the /ws URL with curl or your browser, it will try plain HTTP(S) without asking for websocket upgrade and Apache will complain with the message this bugreport mentions.