One possible reason is that your bank card or the beneficiary’s card has been replaced before its expiry date (following a request via Card Stop for example). If this is the case, you will need to update your payment details.
To do this, open the menu at the top left of your home screen and select 'Payment methods'. Log in and check that the card number set up in your app matches your current card number. If not, delete the old card and add your new Bancontact card.