You’re on the right track, there is either a PHP or JS error that’s causing the AJAX request for the posts to fail. You can try turning on error debugging and logging and then look in the error log to see if there are PHP errors https://codex.wordpress.org/WP_DEBUG, and look in console to see what, if any JS errors there are. If you can figure out where the bug is it will be easier to help.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users