I managed to solve the problem.
The problem were the two “CommonInputBaseControllerData”-Assets (one for GamePad, one for Keybaord and Mouse) which I setup in my project settings under:
“Project Settings > Game > Common Input Settings > Platform Input > Windows > Default > Controller Data”
In the early stage of my experiments with CommonUI, I have created these assets in a folder, but later on moved them into another folder. This movement was not recognized by the Engine. But yet, while searching for the problem, this detail was not recognizable to me. Only after checking the “DefaultGame.ini” file made me realize this detail because the Path was not correct.
So, everything I had to do was refresh the assets by re-choosing them in the project settings.
Hope this helps others in the future.