Don’t know why my first attempt using rollershutter-control didn’t work (maybe the use of position="..." causes problems), using this version of knx.things works like charm.
knx.things
Type rollershutter-control : EG_Terrasse_beide_Markisen_Dummy "EG_Terrasse_beide_Markisen_Dummy" [ upDown="1/2/17", stopMove="1/2/18" ]
More information about this KNX2 behaviour can be found in this thread.