You will receive this error message if you have added Addressfinder as Data Validator tool and setup the keys. Part of the setup also includes adding the domain in Addressfinder Portal which might be missing to cause this error
Refer to the pt 3 in the article how to setup addressfinder
If you are getting Domain not registered error on the internal payment form, i.e. when you use credit card button on the payment form to load the checkout form, then it will be mostly because the internal URL is not added to allow trusted domain list. This URL can be found on the top of the page and will have .force.com (Refer to the image below)
