The 5.2.3.arch1-1-kernel has problems adressing the display port when told to use 4k resolution. It obviously is not a hardware problem since the setup works out-of-the-box when using the 4.19.61-1-lts-kernel. Therefore I stick to the lts-kernel for now.
I am going to dig into the Xorg and kernel logs. Any sufficient help to nail the problem and eventually write a bug report would be highly appreciated.