Fatal error on product list

On loading the product list http://communityresolve.com/coursesworkshops/ we're getting a fatal error. Only affects anonymous users. When it happens it can be temporarily resolved by opening one of the products in Admin and 'update'. Same effect if flushing cache on GoDaddy. BUT the problem crops again when a product page is refreshed or revisited, then the product listing page again fails with fatal error.

Fatal error: MP_Product::on_sale(): The script tried to execute a method or access a property of an incomplete object. Please ensure that the class definition "MP_Product" of the object you are trying to operate on was loaded _before_ unserialize() gets called or provide a __autoload() function to load the class definition in /home/content/n3pnexwpnas01_data02/74/3169274/html/wp-content/plugins/marketpress/includes/common/class-mp-product.php on line 1613

I'm going to have a go at recreating our products (not too many) and deleting previous versions.