I changed the portal language from NL to US and changed the Currency from Euro's to US Dollars.
At least now i'm getting the PayPal checkout page instead of the checkout failure. Unfortunately i can't test it because they don't allow me to transfer money from an account to the same account.
This looks like a format problem of amounts, in different currencies, to me. The Dutch format for amounts is 9.999.999,99 and the United states format is 9,000,000.00
Any help on this topic is appreciated very much.
Regards,
Sidney.