When adding an interface converter to the DisplayPort, my KVM shows no screen.

We don't recommend using interface converters because they may degrade, attenuate the signal and cause the KVM to stop displaying. 
This occurs in dedicated video application, especially for 4K and higher resolution with threshold bandwidth. The converters are considered a tier connection in the installation and consumes some bandwidth. Unless, the interface converters have a chipset built-in. For this information, please contact the manufacturers for details.