Suddenly receiving a Not Acceptable Mod_Security error on checkout.php when dropdown selected

Ray, after you helped be fix a few minor issues with my cart and checkout pages recently everything seems to work well, except I now notice that whenever I select a value from one of the dropdowns on my checkout.php page (prov - country - shipping) the confirm_cart.php is replaced with "Not Acceptable!
An appropriate representation of the requested resource could not be found on this server. This error was generated by Mod_Security." The Continue button remains and, when clicked, processes to PayPal ... but the Not Acceptable is a mystery. Can you think of what might be causing it?
UPDATED - it seems that changing any form value on the checkout page generates the Not Acceptable message. If I refresh the page, the message resets and the form data reverts to original values.

Please provide a url and steps to reproduce.

Sure, Ray .... purple-essentials.com and any process through to checkout.php where selecting any of the three dropdowns will trigger the Not Acceptable response. Thanks!

I fixed the issue by updating to the latest version of: webassist/framework/javascript/ajax.js

It appears to be working now.

Thanks, Ray ... I read the threads that referred to checking the basics first - I guess I better update to the latest Databridge soon :)