Add to Cart button just stopped working

Please can anyone help. My 'add to cart' button has just stopped working on both the stores. Luckily a customer let me know. I have changed theme's to see if that is the problem, but have the same issue on default. Running Opencart 1.5.6.4.

Any help, please.
www.kidzdecor.co.za is the domain for the main store and www.stickups.co.za is the domain for the second store.

I think I fixed it, added www to the url in the https config.

I've had that problem before. I always use just http://mysite or https://mysite.com now. I've found that whichever format you use you have to stick with it for all instances of the domain. I use https://mydomain everywhere now. I don't use the www.