Hi guys,
I believe this is also affecting my installation - appears to be related to the fact that I'm running a reverse proxy to mattermost (443 externally to localhost:8065 - something I need to have, as I require multiple SSL sites on this IP, so I'm depending on SNI). I've currently only spent about 10 minutes looking at this, but it seems like e4de5837d3e91b5b988fa749373683b7f849acf3 is the culprit.
Honestly though, I don't know enough about websocket security to submit a patch for this one.
Edit: It's just occured to me that the host header probably isn't being preserved on my setup, which maybe is what's causing the problem. I'll check tomorrow when I get back to work and update accordingly.