Try an admin-ajax.php skip/bypass rule first. It is possible there are several problems going on since the Request URI Query string is a mess. If this code below does not work then I will need to know the name of the plugin or theme that is causing the error.
1. Copy this admin-ajax.php wp-admin .htaccess bypass / skip rule below to this wp-admin Custom Code text box: – CUSTOM CODE WPADMIN PLUGIN/FILE SKIP RULES.
2. Click the Save wp-admin Custom Code button.
3. Go to the BPS Security Modes page and activate wp-admin folder BulletProof Mode.
# admin-ajax.php skip/bypass rule RewriteCond %{REQUEST_URI} (admin-ajax\.php) [NC] RewriteRule . - [S=2]