User-agent: *
Disallow: /wp-admin/

A default WordPress virtual robots file would usually look like this:

User-agent: *
Disallow: /wp-admin/
Allow: /wp-admin/admin-ajax.php

While I’m not 100% sure it would actually cause a 400 error, you might try adding
Allow: /wp-admin/admin-ajax.php to your robots file and see if the symptom goes away or changes at all.

still struggling with that:)

It’ll help if you can compare network data with a very similar, working Ajax request. Once you identify what is wrong, find the code responsible and alter it to correct the problem. Easier said than done!

How about this?

1) Comment out the middle wp_enqueue_script in init.php —

line 42:
// wp_enqueue_script( ‘wpas-admin-ajax’, admin_url( ‘admin-ajax.php’ ), array(), ‘1’, false );

2) Then inside js/scripts.js —

line 156:
replace
url: WPAS_Ajax.ajaxurl,
with
url: “/wp-admin/admin-ajax.php”,

The error is gone for me and everything seems to be working fine.

Hope this helps.