How to prevent showing background code in Drupal 7 admin?
Sounds like it may be related to (caused by) JQuery Update, similar to what is described in "Cannot edit my views.. I get a JSON string?".

Maybe you want to (temporary) try any of these potential work arounds:


Disable or change the settings within JQuery Update (to see if it solves the issue).
Change the admin theme to any theme included in the Drupal version you downloaded.