Sandbox Paypal Express adds a new empty order?

I've set up a sandbox Paypal Express for my Zen Cart v.1.3.9.h
It seems to work fine with both paypal account and credit card.

But in addition to the original order there is added a new empty order that's already processed in zen cart and it has gift voucher/discount coupon as payment.
It also try to send a confirmation email without using any email address.

Is there something wrong or is this normal when using Paypal sandbox?

I editet one of the empty orders and discovered that it contained a message in the cometary field.

PayPal status: Pending multi_currency @ 12:12:58 Jan 02, 2011 PST
Trans ID:1VR1077971148862B
Amount: 10.50 NOK 

Maybe the empty order is generated because I use another currency than the Paypal sandbox account?

It's telling you that the PayPal account you're processing against is set up to not automatically accept transactions in the currency you're sending them in. In your case, it's not set up to allow NOK without you manually approving the transaction from within your PayPal sandbox.

While it shouldn't create any duplicate orders in that case, you should be able to prevent the problem by either configuring your store to not send transactions in that currency, OR configure your PayPal account to automatically accept all payments in all enabled currencies.