Hi @zippit,
Do you have integrations or anything that might be trying to connect to the WebSocket, other than the Mattermost apps?
In Mattermost 3.5 we added the ability to authenticate with the WebSocket by providing a token over the connection, rather than forcing use of headers or cookies to authenticate before connecting. This error will occur when a WebSocket connects but the client does not authenticate within a set period of time.
Hope that helps!