Willem Hi Willem,
Make sure you check the App Store for updates and download the latest bunq update (there has been a new fix not too long ago).
In the latest version, got to the ‘Me’ menu, click on cards, then click on the card you want to freeze and the click the big plus button. It should give you the option to freeze a card.
If that option is not there, check again that you’re on the latest version of the app and if that’s the case you should contact support.