Check the live feed from the grabber (upper right corner). Try to setup resolution and refresh rate manually. After grabber is disconnected it's necessarily to reboot system to work again.
The grabber displays a vertical bar pattern when there's no signal detected, hence the RGB pattern.
I got it to work with a standard 4K signal, although with a delay that I need to figure out how to fix - I can't set the frames per second to anything above 5.
The bigger problem is that it doesn't look like I can pass a 4K HDR signal to the grabber (it just displays the bar pattern), and the splitter I'm using (link) cannot down-convert an HDR signal to a standard signal without completely washing the image out. Any thoughts on how to make this setup work with HDR?