Your client is too old; please ctrl/cmd-reload the page to get a new one. I have been getting this message for 2 days.
Work is being done on the client and the server. It was a persistent problem for a few hours 2 days ago, but I haven't seen it since.
"CTRL-F5" or "CTRL-R" is the reload command on Windows for (almost) all browsers. On Mac it will be "Cmd-R" AFAIK.