There was an error with the in-App-Purchase.
Requests are very long (>10 sec), and after the abolition of the request window appears several times: "enter your password".
The error is not stable, it appears and disappears on the iphone 3g. for ipad 3 and iPhone 4 does not have this error, even though requests are also very long.
It looks like a network problem, but with the Internet all right. Everything worked fine a few days ago.