User field giving “The results could not be loaded.”

I’m trying to use the User field (https://www.advancedcustomfields.com/resources/user/), but I keep getting “The results could not be loaded.” back.
The console is saying that admin-ajax.php is giving a 404, but I can’t figure out anything beyond that.
It seems to be similar to the issue here (https://support.advancedcustomfields.com/forums/topic/user-and-post-object-fields-not-working-on-front-end-with-acf_form/).

Things I’ve tried:
1. Disabling all unneeded plugins and themes.
2. Enabling debugging and error logging, no PHP errors thrown.
3. Ensured that PHP variables (max post size, etc.) aren’t limiting anything. That shouldn’t be the issue since there are only 3 users on the site.
4. All performance plugins were disabled