Send data to KNX bus using openhab2 api


I’m using the openhab2 api to load data into my OH2 environment using curl with the following url to send the outside temperature stat to the item ‘outsideTemp’:
This works fine - I can see the temperatue in my openhab view.

But: this item is defined es an KNX item with an underlying “thing”. What I need is to bring this temperature to the KNX bus to an specific group address (4/3/1).
My Configuration looks like this:

Thing device outsideactor {
    Type number: outsideTemp [  ga="4/3/1" ]


String outsideTemp  "temp [%.2f °C]"   
       <temperature> { channel="knx:device:ip1:outsideactor:outsideTemp" }

This configuration worked find with openhab 2.1 and an beta-release of knx2 binding.
I was able to display the temperature on my knx devices.
After upgrading to OH2.3 with the stable addon, the temperature does not reach the knx bus anymore.

Any iedas?
Thank you.