Restrictions not saving?

I recently installed this plugin and we plan to launch our first coupons in our webshop soon.

However, we want to restrict the coupons to be only valid for certain products, and when we try to configure this (on the coupon product page), the configuration just doesn’t save – the page is reloaded but the product restriction is gone. Any idea?

The page I need help with: [log in to see the link]