Same Origin Policy

If you're trying to request some content via Ajax.Request, Ajax.Updater, etc., and the request is failing, you may be running afoul of the Same Origin Policy: Your script can only retrieve content from the same "origin" as the page, not from any arbitrary location.

"Origin" in this context includes not just the domain, but also the port and protocol (and so your page served with http cannot make an https request, and vice-versa).

See the linked article for more.