"An AJAX HTTP error occurred.HTTP Result Code: 200" I'm new to Drupal, and I'm working with a Drupal site that someone else wrote. From the admin menu, when I go to Structure -> Pages -> Edit Content -> Gear Menu -> Edit (or Settings), Drupal throws up this error:


  "An AJAX HTTP error occurred. HTTP Result Code: 200 Debugging
  information follows. Path:
  /panels/ajax/editor/edit-pane/panel_context%3Apage-join_step_two%3Apage_join_step_two_panel_context/4
  [...] RESPONSE = nullnullnullnullnullnullnullnullfalseCancelled: Error
  bad character content for element
  'data_key'________nullnullnullfalsenullnull"


The code is also inserting the line 


  "RESPONSE = nullnullnullnullnullnullnullnullfalseCancelled: Error bad
  character content for element 'data_key' ________
  nullnullnullfalsenullnull"


at the bottom of every page.

I know that Firebug can cause the Ajax HTTP 200 error, but I don't have it installed. I've spent hours looking through help pages for this error, and nothing has worked. Can you help? Thanks!
we need to uncomment .htaccess file in php_value auto_append_file none may be it work.
in ajax return value must be defined.if not exist show this kind of error.clear cache.
 one other solution if you working on the local you need to disable newest module than check it[first you need to take DB backup].