Question 4K through KVM switch from one computer but not from other

[Franz47]

After having moved to an ASUS 4K monitor I have the following problem
I have the monitor attached via an 4K-KVM-HDMI-USB switch to two computers.
One Asrock H470M-STX mini-PC with HDMI and Display-Port - works without problems 3840x2160 and 30 Hertz through the KVM-switch with HDMI-cables
The other is a desktop with an MSI B450M Mortar Max motherboard with a Ryzen 5. This one shows via HDMI and KVM-switch the BIOS messages, then Windows logo, rotating circle, then dark screen.

1. It works when attaced directly to the monitor via HDMI-cable
2. I have reduced the frequency to 30Hz under 4K. No effect.
3. It works via connection with display port cable.
4. It has been working with a 1920x1080 60Hz monitor and through the KVM switch.

Any idea why one computer can connect and the other not through the KVM switch although they should have the same HDMI-output?

 

[Ralston18]

Have you tried swapping the KVM input ports?

Meaning = move the min- PC to the desktop inputs and the desktop to the former mini-PC inputs?

Does the problem follow the computer or stay with the KVM port?