According to a report from heise.de, a German news outlet, a bug in HDMI 2.1 outputs on select AV receivers is causing black screens when enabling 4K 120Hz capabilities on Nvidia's Ampere graphics cards or Microsoft's Xbox Series X.
As it turns out, this issue specifically impacts HDMI 2.1 chips from Panasonic, which are installed in Denon, Marantz, and Yamaha AV receivers. For some reason, the chips cannot display the 4K resolution at 120Hz without errors with specific hardware, like the Ampere GPUs and Microsoft's upcoming console.
Fortunately, the issue seems hardware-specific, other gaming devices like the PS5 seem to function perfectly fine. For now, the solution is to reduce your resolution and/or refresh rate to HDMI 2.0b standards (4K 60Hz). Another fix is to connect your AV receiver to your TV's eARC HDMI port and connect your affected device to the TV. This way the AV receiver only deals with processing audio while the TV handles everything else.
New fixes are underway, and Panasonic should be manufacturing new HDMI 2.1 chips that are free of this bug by the middle of 2021. Unfortunately, this also means you'll need to replace your AV receiver if your unit is bugged, and you can't use any of the workarounds already mentioned.