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You have to turn off WP_DEBUG or else make its output go to a file if you want to leave it turned on ( https://codex.wordpress.org/Debugging_in_WordPress ).

The problem is that when you hit Submit on the form, the server returns a little bit of data in JSON format. However, if you have WP_DEBUG turned on, there is an unimportant warning message that occurs and that message gets mixed in with that JSON message and corrupts it. That causes the page to just sit there with the little spinny icon. If you make that warning message go to a file or just not appear at all then the problem goes away. To do that you either have to turn off WP_DEBUG or make its output go to a file.