[lubuntu] ieGeek KVM-Switch goes black after POST

I got an older PC recently and installed Lubuntu 16.04 LTS on it. The plan is to use it alongside a Win 7 Laptop. For the sake of convience I wanted to hook them both on the same display+keyboard and mouse. So I bought the ieGeek 4-Port KVM-Switch (VGA). With the laptop it works like a charm but the PC doesn't.
On boot everything works fine. I can access BIOS, read the POST. After that the screen goes black and stays that way. Not even this loading screen is visible.

But the screen is still powered.
Some seconds later is enters power saving mode due to no signal.
I tried several combinations of cables and monitors. They all work with the laptop but not with the PC. I even added an USB-Hub with own power supply to give more power to the switch which gets his power over USB only.
By coecting the PC directly to the screen, mouse and keyboard it works. If I then pull the Cables and insert the switch into the setup it keeps working fine.
I need help to figure out how to get it to work properly.