This problem happens after you’ve updated NextGen Gallery, when you used the free WP Simple Cart Integration Template available here (scroll down to “How to Integrate NextGen Gallery and WordPress Paypal Shopping Cart Plugins”).
As stated on that page, if you update NextGen Gallery through the WordPress plugin update procedure for any reason, you must copy the
gallery-wp-eStore.php template file back into
/wp-content/plugins/nextgen-gallery/view/ on your server.
This is necessary because when WordPress updates a plugin, it deletes the existing plugin directory and all of its contents, before placing the new updated plugin directory and its contents in. Since
gallery-wp-eStore.php is an add-on template that is not present in the standard NextGen Gallery installation package, it does not get replaced! So, you must remember to do so yourself, every time you update the NextGen Gallery plugin.
Doing this will solve the problem. I had just experienced it myself, and when searching for the answer, Google showed over 300 matches for
" class="shutterset_set_2" > — while the first few were forum posts such as this one looking for the answer, the vast majority appear to be actual WordPress sites displaying that bad code and thus with broken shopping carts, for this very reason!