[geertm]

This story has a happy ending.

According to my credit card company my credit card was okay.
The only thing I could think of was that for some reason B&N was verifying the billing address of my credit card.
As a workaround this week I have bought an USunlocked card. This is a virtual US Discover debit card, registered to an US billing address.
Today the card was loaded, so I replaced the Visa credit card in my B&N account with the new USunlocked card. Unfortunately the same problem.
I could download e-books. I could buy and download free e-books. But as soon as I try to pay for a book, the order is cancelled immediately.

Is my account somehow flagged as international? Or is something wrong with my account? I could only thing of one more solution.
I created another e-mail address, unregistered my Nook and Nook Color, changed the e-mail address to the new one, and registered my Nook Color again.
I then tried again to buy books. Now everything works. No problem paying.
I do not know what solved the problem. The unregistering/registering or the change of e-mail address, but at least everything is all right now.