Paypal Express Checkout telling to try again, despite the money being withdrawn

From a customer we received the message that after an order in the shop she got a message.

Paying through Paypal Express Checkout it said to try again. But when she checked the money was already withdrawn.

She also mentioned that it happened to an friend of hers.

This looks like a serious issue to me. Have you got any idea what might be the problem and what to do about it?

Thanx so far,