I set up my wife’s WordPress site to use UI for WP Simple Paypal Shopping Cart because she doesn’t really understand shortcodes. She’s been chugging along happily with it for months. Now, all of a sudden, the popup window is empty. She uses Firefox and Windows 7. She swears she hasn’t changed anything, but I know her system does automatic updates of Firefox.
Also significant, today I updated WordPress to 3.9.
The plugin works fine in 3 different browsers on my Mac (Chrome, Firefox and Safari). I browsed your code and it seems straight-forward enough. Have you ever seen this problem before? Any clue as to what could be causing it?