Thanks for your reply! I solved the issue a while ago by using an active converter for the displayport side. The reason is that the switch does not support DisplayPort++, so the passive Adapter cannot funktion.
I have to Apple Cinema Displays, with each one DVI-D input. Because I need to use two Computers with these Monitors, I bought a KVM Switch. It has one HDMI in and output and one DisplayPort in and output. The Problem is, that only one of the two Screens are working.
Here is how it is set up...