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 earlier.

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?

 

[Franz47]

Yes I have tried this, too, in the first place. Forgot to mention it. The problem follows the computer.