Editing Visual Editor widget redirects to a front end 404

While editing a layout using Site Origin Page Builder, I click edit on a Visual Editor widget, and I am immediately redirected to the front end with a 404, and the same URL.

In the Chrome console I see:

VM2458:1 POST https://www.example.com/wp-admin/admin-ajax.php?_panelsnonce=removed8&_fs_blog_admin=true 404 (Not Found)

Previously, in /wp-admin/.htaccess we had:

order deny,allow
deny from all
allow from xxx.xxx.xxx.xxx
<Files admin-ajax.php>
     Order allow,deny
     Allow from all
     Satisfy any
</Files>

Commenting out all that code does not resolve the problem.

Help appreciated.