I thnk that you need to “look underneath these other toadstools.” It is actually unreasonable to think that something “just happened yesterday” to the material that is being received from the AJAX call. If your shop maintains version-control as it should, pore through the list of recent commits. Look, with an open mind, for anything that this material may “touch,” that has been updated in any way whatsoever during the period of time since your “last known good” state. Also, clear the browser caches and methodically test the behavior to “box it in” on various available computer systems and browser types. The error-messages and (e.g. FireBug) debugging outputs available on different browsers might give you an important clue.