Error `400` on https://www.example.com/wordpress/wp-admin/admin-ajax.php:

I’m getting error 400 on https://www.example.com/wordpress/wp-admin/admin-ajax.php:
Request Method: POST
Status Code: 400

It happens when I:

  • try to install or delete a plugin
  • try to update a post
  • I also noticed that when I’m on the post overview page (https://www.example.com/wordpress/wp-admin/edit.php), WordPress keeps logging load-scripts.php?c=0&load(chunk_0)=jquery-core,jquery-migrate,utils&ver=5.7.2:2 POST https://www.example.com/wordpress/wp-admin/admin-ajax.php 400
    every 15 seconds to the Google Chrome developer console.

What I tried:

  • added IUSR, NETWORK SERVICE, IIS_IUSERS with full permissions on wp_content folder
  • deactived all my plugins
  • added debug to wp-config.php, but very little and nothing of interest gets logged
  • Set permalinks to Plain
  • plugin Health Check (https://wordpress.org/plugins/health-check/) reports The REST API did not process the context query parameter correctly.
  • Tried reinstalling WordPress and restore my database

What else can I try?

I checked here:

https://wordpress.org/support/topic/cant-update-plugins-or-theme-cant-add-media-pages-or-posts/
https://wordpress.org/support/topic/the-backup-of-this-post-in-your-browser/
https://wordpress.org/support/topic/page-post-updating-failed-wordpress-5-2-rest-api-context-query-parameter/