Error message on Apple TV: "HDMI cable or connection doesn't support HDCP"

I recently upgraded from an Apple TV Gen 4 to an Apple TV 4K (I have a 4K TV, so thought it'd be nice to utilise my premium Netflix subscription).

Same HDMI cables, same TV, different Apple TV...I'm getting the following message when it comes to watching anything on Netflix - not just 4K - even previews while scrolling through the available titles:

"This video can't be played. It looks like your HDMI cable or connection doesn't support HDCP, which is required to play this type of video. Try reconnecting the HDMI cable from Apple TV to your TV and play this video again".

What I've tried so far, with only temporary success:

- Reboot Apple TV

- Reboot TV

- Unplug and replug HDMI cable

- Use different HDMI cables

- Purchase new HDMI cable that allow larger data transfer (21 Gbps)

- Try different HDMI input on TV

- Update TV's software

- "Reset and Update" Apple TV

- Tried various formats at "Video and Audio" Apple TV settings, even "1080p SDR" but I have no issues (flickering screen etc) with "4K SDR 60Hz". Still, no Netflix though no matter which one I've chosen.

Something fishy: Every time I've tried "Check Cable" at "Video and Audio" Apple TV settings, with any HDMI cable (expensive Monster included) and at any HDMI TV input, it takes me to the:

"Now, check your screen, If you see a clear picture, select OK. Select Cancel if you see sparkles, static or other display problems."

The picture is clear, but it won't let me select the "OK". I've tried the normal remote, the Apple TV app and my universal Logitech remote control.

It then takes me to the: "There's a problem with your HDMI cable. Older cables might not be compatible with your system. Long or poorly shielded cables may also cause problems." and it just stays there - it won't allow me to select "OK" and move on. Only solution is either rebooting the Apple TV or pressing over 30 times the Menu or Home button on the remote. I can then see the main menu on the background, and the "OK" button suddenly becomes responsive again.

Any suggestions please?