The usual culprit for socket.io issues is usually not having sticky sessions. Socket.io requires sticky sessions so requests from a client always go to the same backends.

On that note, a fully written out "demo" config.json file (or even a couple of them, one for Mongo and one for Redis) would be a nice to have in the documentation article about the nodebb config (in addition to the explanations of what each entry does). I'm willing to throw that together if others would find it useful.