Had a Roku Ultra since late 2018 and intermittently (for apx 2 years!) I get HDCP errors . . . except that the typical HDCP trouble-screen wouldn't appear.
Finally figured it out . . . info for others who may have the same HDMI cables. The problem was due to the Sewell "solid-state" HDMI cables . . they are very small (dia) and have active electronics in each end of the cable (also are SOURCE and TV end sensitive).
They were the main problem so to fix:
1) replaced the HDMI cable with a 'standard' HDMI cable;
2) Programmed the Roku TV video to 1080p rather than "auto-detect";
3) Received a new sw/fw update from Roku.
So the video is smooth as glass. . . . no more HDCP errors.