WP e-commerce – Can’t add products to cart

When I add a product to cart, it shows up in the cart, but vanishes on a page refresh/when adding a new product. There are 0 rows in the WPSC_TABLE_CART_CONTENTS table, so doesn’t look like the script ever inserts them there. Session ID remains the same.

I tried it without JavaScript on, and got a line of commented PHP code at the top of the page but products were still not added to cart.

I’m running WP 2.8.1 and WP eCommerce 3.7 and slightly (only CSS) modified Vanilla Cart theme (same problems happened with the default theme, too).

Any help would be very much appreciated. Thanks!