Your access token is not authorized: Subject not found

I get the error message “Your access token is not authorized: Subject not found.” when I send a GET request (https://api.nuki.io/smartlock/XXX/auth), but only for a few of the smartlocks in the user’s account. What could be the reason for this?

The user told me everything was set up with the same account (otherweise I couldn’t see them in the list of smartlocks, I guess) and device. Thanks for any ideas!