For me, when on stackexchange.com I enter something into the
search all sites box, the following page simply shows "Loading" in the content area. Is this just me or is it buggy?
I figured out that Ghostery blocks "Google AJAX Search API". (Of course, in your case something else may be responsible, like a firewall.)
When it's blocked, the page appears to never load. Unblocking the "Google AJAX Search API" will allow it to load.