It’s a week of solutions as I also seem to have found a fix for the 4K Roku and TV issues. We had upgraded the main TV to a 4K HDR model, using a Roku Streaming Stick +, capable of 4K HDR. However, after numerous attempts, 4K content continued to give a HDCP error, indicating some sort of hardware issue.
Eventually I had settled on using an older Roku model, and the built in smart tv based Plex as a solution, even though that was less than ideal (but I didn’t want to accept defeat).
Then, while browsing for unrelated hints, I spotted a note that said that an underpowered 4K device would give similar errors. This caused me to think to plug the Roku into the wall, instead of the tv USB port, even though the Roku never indicated a low power condition.
As soon as I did that, the errors went away. After testing numerous video files and 4K YouTube, I am 90% sure that low power was the cause, and it is now fixed. Another technical issue resolved and cleared off my to do list.