When the user manages to send an invalid IBAN to the server as part of creating a donation, the controller can't render the error message, because it makes the assumption that the values it gets are strings. The IBAN, however, is an IBAN class.
Recoverable fatal error: Object of class WMDE\Fundraising\PaymentContext\Domain\Model\Iban could not be converted to string in /usr/share/nginx/www/spenden.wikimedia.de/current/app/Controllers/AddDonationController.php on line 252
This error has low priority because this can only happen when the client-side validation fails. But it points at a possible architecture violation (using a domain object in the presentation layer).