Fix some errors on our website

We just experienced today that some customer are not able to add products to the basket in our online store. Some product can only be added from the frontpage, others only from the product page and some not possible at all to add a product. We are unsure what caused this issue and the issues seems to be different from browser to browser and from domaine to domaine. We have 3 active domains: [login to view URL] [login to view URL] [login to view URL] The freelancer needs to have time just now as it is urgent for our business that the issues is fixed asap.