Updating an edited playlist gives Error 400

I recently began having trouble with the Cue plugin. I’ve been adding a track to a playlist on a weekly basis for over a year without trouble. Now, when I add a track to a playlist and click on the Update button, nothing happens. No spinner animation by the button, or any indication on the page that the update has succeeded or failed. Checking Chrome and Firefox’s console, I’ve found that I get this error: https://bsumc.info/wp-admin/admin-ajax.php 400 (Bad Request). If I open a playlist, don’t make any changes, and click update, it goes through.

I’ve disabled cacheing, in case that was the cause. As far as I know, there haven’t been any major updates to the site. It’s hosted on WPEngine, so there may have been a background update I wasn’t aware of. Any ideas on how to start debugging this? Thanks in advance.