User blog comment:Rappy 4187/Technical Update: May 25, 2015/@comment-126761-20150527140850

Quick note regarding current issues with Chat: we're currently looking into what is causing it to fail.

Update: it looks like we know the cause - it's due to socket.io's security certificate expiring (a feature used by Chat). Unfortunately, this is likely out of our hands - someone messed up on their side. This also means I cannot provide an ETA on a fix, frustratingly.

A quick glance at Twitter mentions of socket.io shows a lot of other people seeing similar issues on other sites.

Apologies that this is affecting you guys!

Update 2: a fix for the issue has just gone live. Refresh your chats to join! For those interested, we simply switched to loading the script locally, rather than rely on socket.io's version - thus avoiding the security certificate error.