Have an issue with Smush on this site:
We also have a staging version o fht is site staging.tanzecampus.com and issue not occurring there. We've tried deactivating the Smush plugin on the live site and the issue goes away. We've activated it again.
The only thing useful I can give you is it relates to the media library when you click an attachment and it tries to show the details in the modal box. An AJAX request is sent but it comes back with a 400 error. The only difference I see from staging to live is the type of "action" query string being requested. Staging requests "get-attachment" and the live site (which is broken) requests "smush_get_attachment_details"