The error will popup when an invalid ajax response was sent back by the server.
Open the page in Chrome, hit the F12 key and click the Console tab.
Use for in the same manner the generated the error. Chrome should show additional error info that may help. Let me know what those messages are.
On the rare occasion when this error does crop up, it is always because another plugin is creating a PHP notice or warning during an ajax request. It is standard practice to disable these on live sites, here’s a link to a tutorial which covers how to do this.
Hope this helps, let me know.