I found the bug: It has nothing to do with Assistance Option. There must be a bug within MSFS: Simply, if you switch on developer mode and change to another plane, the Boeing 747-8 it works with the ATC responding…If you then re-change the aircraft to the A320neo f.ex, it still works…but you have to change the aircraft by the developer mode: Tools → Aircrafr selector, not via the main menu…