Socket timeout while chat client is connecting

This sounds pretty coincidental to me.

If the socket connection is timing out, its because the response coming back from the server is taking a long time.

If this is thousands of users on the same network (i.e. you're on a corporate LAN), then looking at your network management tools for bottlenecks between the client and host would be a good next step (as well as looking at load and network performance on the server itself).

If the issue is on the Internet over a WAN to a third-party service, then network performance along a route is going to be highly dependent on geography and network topology, and you're not going to have any insight into their server performance.  The content provider is best positioned to troubleshoot that issue.

Off the top of my head, I can't think of anything specifically aimed at socket communication that's shipped recently, and we definitely wouldn't have made a huge change to timeout thresholds for this specific reason.

My guess is that there are a handful of people that were skating by with network conditions that were marginal (sketchy/overutilized wifi links, poorly-peered ISPs, etc), and something has pushed them over the edge.  More than likely, that's a small performance cost related to a security mitigation, or a change to the host browser (both Flash Player and the browsers are constantly hardening against emerging threats).

The optimal solution isn't going to be to try and just nudge them back to the right side of that line, but to find and address the underlying latency such that their network performance is solid.

You're really going to have to look at the network performance (via Wireshark, etc) on an affected client to understand what's going on.  I think you'll probably see slow responses coming back from requests when you encounter the timeouts.  At that point, you can rule out Flash Player.