Black window on Apple TV+ content

My Mac is no longer able to display any Apple TV+ content in-app. I only get a black screen with uninterrupted audio. I'm guessing it's a HDCP issue - one of countless I experience with Apple apps (Safari and the like) but never with other apps (like Google Chrome). Any ideas how to fix it? Being a Mac Mini the only option is for an external monitor and the monitor is connected to the HDMI port. I think I remember it working before.