Hi Mike,
Theres a 99% chance this just means you haven't copied the latest web folder to your app.
That's the usual case where I see it.
The other 1% would be because the "reply" from the server is not valid xHtml. In other words when you press whatever button, or row, or whatever that causes the Busy gif to appear, then the reply coming to the browser is not correct. If this happens the busy won't disappear.
Cheers
Bruce