Can't set featured image

This client is a salon that offers a number of unique products for sale in the salon. These products are also available at an external website. I have tried out several ecommerce plugins but this one looks quite promising. She wants to show the products online in a catalog format without a shopping cart.

My problem is that I can’t set the product image. In Chrome, when I use the <ctrl> <shift> <J> to bring up the console and then try to set the image, I get a 404 not found for this specific snippet in admin-ajax.php:

POST 404 (Not Found) jquery.min.js:4
send jquery.min.js:4
f.extend.ajax jquery.min.js:4
f.(anonymous function) jquery.min.js:4
WPSetAsThumbnail set-post-thumbnail.js:1

The rest of the script runs fine. I have increased the memory limit to 128M and my server provider (I am a reseller) has white listed the domain in mod_security. I have cleared my browser cache. I still have the problem!

I would really, really appreciate any help. I am new to the community but truly love the plugins and it’s very frustrating for both me and my clients when I can’t get them to work properly!