Hi Timo, and welcome to the forum 
The box does not support the simplified “lock” and “unlock” actions.
To unlock/lock a door using a box, please use the smartlock/{smartlockId}/action endpoint, and pass the appropriate action in the request body (see https://api.nuki.io/#/Smartlock/SmartlockActionResource_post_post).
The available actions are detailed in “M. Smart Lock Actions” within the API documentation, but for quick reference for any one else finding this thread, “1” is unlock.