Product Enquiry Button shows but no form (Lite version)

Hi the Product Enquiry Button is great, but on a random basis it’s not working. You press the button and a small box shows up with ‘-1’ in it. I’ve ‘applied this to logged in users’. Some days it works, some days not, some days it works on some products but not others. I’ve tried it in Safari and Firefox. Any ideas? It’s a Sliding Door Theme. I haven’t upgraded WP E-Commerce from 3.8.9.5 to 3.8.12.1 as last time I updated it crashed the site very badly and took too long to repair.

https://www.lalunadeplata.co.uk/products-page/bangles-and-bracelets-cuffs/newsterling-silver-indalo-almeria-man-bracelet-or-pendant-or-earrings/

Thanks very much if you have any thoughts. Regards, Lesley

https://www.ads-software.com/plugins/wp-e-commerce-catalog-visibility-and-email-inquiry/