I'm glad you're enjoying the book!
A first thing to try would be to check that the latest version of the code has the same problem. You can find it at http://github.com/metajack/profxmpp
The next thing to check is what is the BOSH service URL you are using? Is it http://bosh.metajack.im:5280/xmpp-httpbind?
Or is it something else?
Next, remember that flXHR-using code must be run from an http:// URL. It won't work if you run it file file:// URLs due to Flash security policy.
It sounds like a browser or flash security policy violation.