I fixed it by using 4K HDR 60Hz TV.
Streaming Stick+ 3810 kept picking 4K 30Hz TV display mode which kept getting the HDCP error. I then tried 4K HDR 30Hz TV because it passed all Roku display checks but it kept getting the HDCP error as well. I then tried 4K HDR 60Hz TV even though it didn't pass the last display check but I told it to Force it.
Seems like Roku is having a hard time figuring out what display modes are compatible with my Sony KD-50X690E.
I have the same TV. Have you tried using the roku on HDMI 1? Mine will not work at all on HDMI1. I get the HDCP error on everything and sound is non-existent at all. I tried so many times all the troubleshooting steps of unplugging, powering off, factory resetting, using different cables, nothing works. Roku support is absolutely ATROCIOUS they they spew the same crap. One even told me to just go buy a new TV. Seriously? Anyway, since I got mine through Amazon, they accepted my return and I got another to try. It worked right away and I had sound the whole way through the set up until it auto updated then it went dead again.
I think the current update for the Roku Stick+ is screwing things up. I think that despite whatever non-4k setting you have it at, the Roku still sends out HDCP 2.2 signals so my HDMI 1, which doesn't support 2.2, just stops working properly. It's SO freaking frustrating.