Custom field (text) required with alert message your order is empty

Hi,

I think that the issue is that the HikaShop cart notification plugin has been disabled. Because of that, and because there is no cart module on the page, the system has no means of notifying the user of a problem on the current page. So it redirects to the checkout which then sees that the cart is empty and redirects to the "URL where to redirect when the cart is empty" of the configuration.
So check on that notification plugin. You want to have it published so that it can handle the display of errors/successes and redirect to the checkout or not if appropriate.