Not every screen works on a Dell Dock WD15? (via HDMI)?

Question

This is the situation:

Hardware: Dell XPS 15 9560 Laptop connected to a Dell Dock WD15 over USB type C.

I have an external monitor (Iiyama) connected via HDMI (via the docking station). It has worked successfully for 2 months. Suddenly when I was working, the external monitor turned black and I couldn't get it to work anymore. I tried many reboots / (un-)pluggings and driver updates). When I put the HDMI cable directly into my laptop instead of via the docking station, the external monitor (Iiyama) would work again. An issue with the docking station, one would assert..

However just for testing purposes: I used another monitor (AOC) I had lying around, so I connected the external monitor (AOC) to the docking station over HDMI, and again my laptop to the docking station. And it just worked.

So monitor A works only via direct HDMI in laptop, Monitor B works both via direct HDMI and via docking.

Does anybody have an explanation? Should I buy a new external monitor? I can only find reasons in the direction of power leakage that the docking station can't handle?

Thanks in advance.