I ran all available updates, both operating system and drivers. Nothing changed.
Please note that the customer uses three different docking stations, two a.m. Fujitsu and one Lenovo ThinkPad Basic USB 3.0 Dock (4X10A06688). No way to make backlight on when it's connected. The local service partner came and replaced something inside the notebook (I was not present, I think he changed the motherboard) but as you can see the problem remains. Note that the old U939X worked fine with all the available docking stations.
What else can I do?