Hi,
We would need you to get in touch with our team through our website for such a technical issue. At a first glance, the result of the AJAX operation is returning a 404 error message for not found, which is converted into that static error string within our modal box/window.
It may be possible that some Shortcodes are missing in your configuration, and so the AJAX request URI cannot be routed properly.
However, these are pretty technical questions and without being able to actually see the link showing that error message, we are unable to give you a precise solution. That is why I suggested to contact us through our website even if you are not using the commercial version of the plugin so that we can assist you better. Thank you.