WP /wp-admin/admin-ajax.php produces 404

Our site has been working fine in the last week. Today, I can’t add a new page to our menu. Clicking “Add to Menu” just produces a spinning icon that never disappears.

If I look at the Chrome console, I see:

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

There are no restrictions on /wp-admin or /wp-admin/admin-ajax.php in /.htaccess or /wp-admin/.htaccess.

I’ve Googled for this problem but most Q&A post relate to custom ajax calls.

In our case we’re not making custom ajax calls.

Troubleshooting steps appreciated.

PS – I am waiting on the web host to check if mod_security is causing this issue.

Your /wp-admin/admin-ajax.php might have been modified by another php file or plugin. Try downloading a copy of your wordpress version and overwrite admin-ajax.php with the original source file.
If that doesn’t work, try deactivating your plugins (or theme) one by one until you notice a difference.