We have upgraded the server that supports the simulator client program. Version 427 and later are using this new server.
If you are running version 426 or earlier, the site will not think it is connected when you try to simulate, or it might error and/or automatically disconnect. Simply close and restart your clients, and they will automatically update to version 427 (or higher) and reconnect.
Please let us know immediately if you run into any issues with this update to the client program.