Hello, The problem is solved. The monitor with Type-C Cable is now connected to the second Port on the changeable part of the Dock. The Port is Typ-C-Port with Thunderbolt 3.
Hello,  I have a Problem since I reassemble my working place with the same Hardware Configuration than the 1.5 Years before. From the beginning I worked with 2 UHD monitors in 60Hz. After reassemble the place, both only work in 30Hz refresh rate. After I contact the support, we changed for testing, the resolution to a lower one. Now I have one Monitor in 3860x2160 60Hz (Connected to DP) and one with 1920x1080 60Hz (Connected to Type-C DP Port). The monitors, the dock, the XPS, the cables are exactly the same before and are connected to the same port at the dock.   Maybe someone can help me to fix the issue.
Resolution and Refresh Issue with XPS 7590 and WD19TB