PayPal Express Checkout won't accept my credit cards

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: https://www.nordkom.eu

DML73:

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"
If the message is occurring on the PayPal site, you'll have to take up the matter with PayPal.

Personally I'm getting fed up with PayPal. I've lost orders because of the unhelpful errors the payment process presents.

Here's what PayPal says about denied credit cards (I've had my share of these).

** Question : Why was my credit card denied?**

Answer : PayPal's process for accepting cards is more stringent and complicated. This process helps prevent the use of stolen and unauthorized cards. Since this process is very complex, it does occasionally affect valid cards. The denial of your credit card is not reflective of your credit worthiness. In cases where a valid card fails our system checks, we suggest that you try another credit card in the system.
Helpful huh!? I sat with a friend last week and tried about four valid cards. No luck.
DrByte is correct in principle, but I tried calling PayPal 888 221 1161. It seems the number doesn't work.

DrByte:

If the message is occurring on the PayPal site, you'll have to take up the matter with PayPal.

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.

"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.

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

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

I found I had to set my 'ZenCart' billing, shipping and the address I put in Paypal must all match for it to work..