Occasional HDCP error with Apple TV 4K (5th gen) on Amazon Prime Video

I started watching the short 8 episode series 'Truth Seekers' on Amazon Prime Video. First two episodes played fine but episode 3 refused to play and threw up this HDCP problem.

This annoying problem which prevents me seeing the episode also occurs with episodes 6, 7 and 8. My ATV is connected to a receiver for cinema sound.

I went down all the usual avenues of restarting, resetting, changing the HDMI cable (yawn, yawn) but the problem persists. The problem does reside with Apple TV as playing from the Mac is fine as well as playing from the Amazon Prime app built in to the TV.

Why should such a problem suddenly rear its head after 3.5 years of trouble-free viewing and in such a selective manner?

It's odd that the next 6th gen Apple TV is just around the corner and now this happens 🤨. I am very skeptical and are not getting into the HDMI splitter solution (I shouldn't have to) so anyone got any useful technical reasons to offer?.. could Apple be steering me towards upgrading to gen 6?