Mattermost websocket error

Hello,

We're on gitlab-ee=8.11.2 (Omnibus) and we still get the following error (this should've gotten fixed on the last update):

[2016/08/27 11:24:22 EDT] [EROR] websocket connect err: websocket: could not find connection header with token 'upgrade'
[2016/08/27 11:24:22 EDT] [EROR] /api/v3/users/websocket:connect code=500 rid=6ipaubudy7f4ueddhr3a371j8w uid=8z1d45uhotnuifu5pqb93gp5ca ip=10.7.61.152, 10.96.14.126 Failed to upgrade websocket connection [details: ]

On these mattermost_nginx configs:

mattermost_nginx['listen_port'] = 1234
mattermost_nginx['listen_https'] = false

mattermost_nginx['proxy_set_headers'] = {
  "Host" => "$http_host",
  "X-Real-IP" => "$remote_addr",
  "X-Forwarded-For" => "$proxy_add_x_forwarded_for",
  "X-Frame-Options" => "SAMEORIGIN",
  "X-Forwarded-Proto" => "https",
  "X-Forwarded-Ssl" => "on"
}

Any ideas why?