If you inspect the JSON response from that request, you should see something like this:
"id" : "uploadFormFor_REGULAR_FILES_DATABASE_RECORD_FIELD__484178052475846997_0"
, "file_type" : ""
, "error" : "Photo files are not allowed."
The issue is that photos aren't allowed in that field. Has somebody changed the configuration of your database recently? Look here:
multiplePhotoUpload field you have only supports regular files and documents. As it is configured right now, it will not allow audio, video, or photo files to be uploaded, which is why the error is happening. To fix, you should add a new field of type Photo instead. You'll probably want to remove the old/incorrect field and give the new Photo field the same name.
The reason your staging site works is because it is properly configured to allow photos:
See "Multiple Photos" listed as the type vs. "Multiple Files" on your production site.
Hope that helps!