Okay, so it basically works.
Try installing a small delay. For example:
when fenster-kinderzimmer is opened set mode of max-thermostat-kinderzimmer to "manu" and set temp of max-thermostat-kinderzimmer to 4.5 after 10 seconds
But as I said, this rule works for me.
Maybe there’s a problem connecting to the cube. I’ve had that before. Restarting the cube helped.