ajax calls from remote site stop working

This is a strange situation (to me at least). We have a function that is called by a third party site via ajax. It is basically a data feed. Works great for days on end. Then suddenly it stops. If I toggle the iThemes Security “Banned Users” on and off, it works again (for a while).

I have tried adding the remote IP address to the whitelist settings (IP address is static – does not change). I don’t see the remote IP address in the security log either.

We’ve added the following to the .htaccess file and it doesn’t help either:

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

I would love to open a ticket directly with the plugin author but they require a purchased product license.

Has anyone else run into this issue and found a solution?

Thanks!