Hi Willem!
Please note we’ve already reported this bug to our developers and they are working towards fixing this issue asap.
As I can't provide you with an ETA, there is currently a workaround for this: you can cancel and block the card permanently, and then order a complete new card, this will allow you to change the name to a new one under 20 characters. You can find this info on: https://together.bunq.com/d/3509
If you get charged for this, simply contact our support through https://www.bunq.com/link/support and we will refund you this amount.