[Solved] Advanced File Attachments doesn't work with WordPress 5.9

Until the problem is solved you can replace the core.min.js (v1.13.0)
with the older core.min.js (v1.12.1)

wp-includes/js/jquery/ui/core.min.js

then Advanced File Attachments works again

but make a backup of core.min.js (v1.13.0) to restore it.

core.min.js 1.12.1

I added the following code and that solved it!

add_action( 'wp_head', function(){
	wp_deregister_script( 'jquery-ui-core' );
	wp_register_script( 'jquery-ui-core', 'https://ajax.googleapis.com/ajax/libs/jqueryui/1.12.1/jquery-ui.min.js', ['jquery'], '1.12.1' );
}, 0);

We're currently working on the addon update. Once the addon update is release, you should remove/revert all custom changes you've done in your website.

The update is released. Please update wpForo Advanced Attachments addon to the just released 2.1.2 version. Then delete all caches and check it again. If you have CSS/JS optimizer plugin, don't forget to reset that cache as well.

Works like a charm, thank you!