When I enable HDMI IN full screen mode I always get a black screen. I have tried with and without an HDMI splitter - no difference. That means I can make it work on a very, very few occasions - with or without a splitter.
When I enable HDMI IN PiP mode it always works, with or without a splitter.
Its a Windows 10 PC that is connected to the HDMI IN socket.
Will it help to update the image to a newer version?

My STB is an AX Quadbox HD2400 (Mutant HD2400)