PayPal Express Checkout won't accept my credit cards

[DML73]

Zen Cart ver. 1.3.8.a.
I installed the PayPal Express Checkout module and entered all the required information, an it is live.

I add a product to my cart and proceed successfully to PayPal Express and enter credit card number and related info.
When I click 'Review Payment' I get this error from PayPal:

"The card you entered cannot be used for this payment. Please enter a different credit or debit card number"

I tried several different valid credit cards with no luck.

I would think the the problem lies with PayPal but they tell me that they cannot help because they have not received any attempted transactions.

VISA, Mastercard and AMEX are enabled in Configuration -> Credit Cards.

I tested the communication succesfully with the cURL Test and IPN Test.

What could be the problem?
Website: www.nordkom.eu