Variations causing lightbox to hang

I have three products with variations.
On two of them, when you try to choose a variation the lightbox appears but the wheel just spins eternally:
On the third it works as expected:

I've tried changing the setting in the first two to match the third.
I've tried creating a brand new product with variations and I get the same hanging behaviour.
I have reinstalled the plugin files.
I have checked in Firefox, IE and Chrome: same behaviour in all of them.

Please help, it's stopping people from using the shop completely, these are some of our most popular products.