Paypal: Pending payments with Express Checkout

I have implemented Paypal Express Checkout on my website. So far everything seems to work in my sandbox, except that the payments are not processed. Every payment is "pending".

The only reason I get is

    RECEIVING_PREFERENCE_MANDATES_MANUAL_ACTION

in the API response after executing the payment via API.

I cannot find any documentation on the reason, does anyone know how to fix it?