Not sure if something is wrong with the KVM or if it's macOS 27 DB2. Last time I used it was with DB1. It worked fine then. Now macOS isn't recognizing the HDMI display of the remote KVM. Switching the cable didn't help.
I figured it out. I have an Apple Studio Display as primary display. With the KVM set to 2560x1440 60Hz GLKVM it doesn't work, but if I change it to 2560x1440 60GHz ViewSonic it works.