I am struggling a bit with something that should work straight out of the box.

I have finally replaced my aging and creaky vista box with a nice new machine (i7, solid state drive, Win10). Since I still want to access it, at least until I have migrated everything off it and replaced some legacy software, I have put the new and old boxes side by side and pulled a 4-port DVI KVM-switch out of the garage (Alltech MP4711 if that's useful).

Both boxes have been connected by plugging the DVI cable into the graphics out and the USB into a USB slot etc. The other end of the cables have been connected to the KVM; as have the keyboard, mouse and screen.

When I start the machine, The right lights appear on the KVM to show that I have active machines on ports 1 and 2. The keyboard and mouse light up in the normal fashion.

The Vista box works fine. When I push the right button, its desktop appears on the screen and the keyboard and mouse work correctly. I'm using that box through the KVM to type this right now.

The new box doesn't work properly. I get a splash screen and windows boot animation appear on the screen, then it goes blank, with nothing.

The screen works just fine when I connect it directly to the same graphics port on the new machine, but not when connected through the KVM. And it's clearly connected correctly or I wouldn't get the startup splash screen. The same keyboard and mouse also work fine when directly connected, but I have no idea whether they do through the KVM (as I can't see anything).

Any suggestions gratefully received.