I am encountering the very same problem! What is strange is that it affects some, but not all customers. So far it has affected ~ 50% of the customers who tried to purchase my products. Below is a screen-shot sent to me by a customer.
As I mentioned, it seems to affect
certain customers, not certain purchase buttons: some people purchased product X from my site without any problems, while other people were unable to buy it.
I asked the customer who sent me this screen-shot, to try to purchase other products from my site. He faced the same problem, no matter the product he tried to buy.
Also, this doesn't seem to be related to certain countries.
Although I subscribed to Hikashop Business 3 since January, I am still using 2.6.4 on my site.
Jerome and Nicolas: My opinion is that you must investigate this very serious issue. We rely on Hikashop to do business and at the moment our businesses are screwed up. I bet that this issue has affected many of your customers.
Thanks
Panos