Getting HDCP error playing iTunes movies on iMac screen

This has me totally stumped.

Anytime I try to play a purchased iTunes movie, it'll play for roughly 35 seconds after which the video goes dark but audio continues normally. An error message pops up saying "This movie can be played only on displays that support HDCP (High-bandwidth Digital Content Protection)."

Ofcourse, playing the SD version is fine.

How the heck does Apple's own iMac screen cough up a HDCP incompatibility error???

Stats: 2019 iMac 21.5-in 4K Retina, Apple TV app, Big Sur 11.2.1

I tried:

Reinstalled Big Sur, reset SMC and NVRAM, there is no screen capturing software, and booting in Safe mode prevents movie from playing altogether.

Any tips before I call Apple support?

Thanks.