New mail yesterday from ticketone.it :
with reference to your request to convert the voucher related to the Paul McCartney event, we inform you that the data you entered was incorrect or insufficient. We kindly ask you to check with absolute precision the correctness of the IBAN indicated. In case of no reply or incorrect IBAN, the refund cannot be processed and we will not proceed with a further request.
It's incredible, they'll try every thing to do not refund the tickets ! One of my friend have receive yesterday the same response. It's strange that him and me entered wrong IBAN on the same time ! I'm really stunned.