Hi Wouter,
It is not possible to enable scheduled events in sandboxes, since we cannot guarantee they will be actually executed. Scheduled events will not be run if the app sleeps, and apps will be put to sleep after they have not been visited for a while to save resources (they are free after all ;-))
The sandbox does not have scheduled events. Thats because the environment goes to sleep with no active users. And in sleep mode scheduled events do not work.
Regards,
Ronald