Dear all,
Try downgrading jQuery from 1.11 (comes with WP 4.0) to an earlier version which your website used to work with.
In my case I had a WP site which after I downgraded jquery to 1.7 everything worked flawlessly.
This happens because some jQuery 1.7 functions are not in 1.11 and an error spawns thus ruining most of jQuery based operation in the backend!
Hope this helps someone!
