Redeeming invitation code failed

Hi all
Using the Airbnb Nuki integration, it usually works perfectly well but, yesterday a guest and all his group tried to enter the credentials and got this pop up message on his phone “Redeeming invitation code failed”.
I tried to send him manually some new codes I created, but the same happened.
He deleted the app on his phone, redownloaded it, but still the same message.
Any idea why this happened and how to solve it ?
thanks a lot