add failed

hi Thimo, the plugin is getting better and better. I have a problem with one of my sites http://www.anxietees.co.uk. all tees when added to basket fail. i also not that the size and colour selectors do not appear on the page, is this connected. I have rebuilt the cache, un installed the plugin and reinstalled, but nothing has fixed the problem, can you help possibly? many thanks, steve

https://wordpress.org/plugins/wp-spreadplugin/