define( 'SCRIPT_DEBUG', true); in place, this causes WP and most plugins (that follow the rules) including ACF to load the unminified version of JS files. This means that there is a difference between the minified and unminified version of some script that that the unminified version in some script is causing an error or otherwise interfering with ACF’s AJAX request.
In order to figure out what’s going on you need to figure out what is causing the interference. First remove the line from your config and then deactivate other plugins and maybe switch to a default WP theme to see if you can narrow it down.
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