I'm still having the same issue.
Go to play content on my ATV3, and I'll occasionally get "This content requires HDCP...", etc.
Now, the difference for me is restarting the ATV actually works and I'm able to play (any) content. But then, a day or so later, the problem (error message) reoccurs and I have to restart the ATV all over again. So not a huge issue, but it's definitely mighty annoying. Particularly since one has to wait quite a while after clicking Play before the error message actually appears and you know the content isn't going to play... then quite a while for the whole reset process. By the time it's all over, you've lost the interest in playing the content in the first place ;-)
I'm already using gold-plated cables, and have tried switching HDMI ports on the TV.
I found myself with a rare stretch of time when I could actually sit down and troubleshoot, and it seems unplugging and replugging the HDMI cable also works in my case... but I'm not going to get up everytime to unplug a cable or teach SWMBO what to do. We'll stick with restarting, for now.
The fact that it's so random when it occurs, and the fact that once I restart everything works fine for a while, is the most frustrating part. Seems like once it's working, it should work from then on. Unless my LG TV is somehow timing out it's HDMI ports every 48 hours or whatever and resetting the handshake (thus reintroducing the error). Would that make sense?? I looked for any settings in the TV that might point to this and didn't see any.
From what I've read, HDCP doesn't work worth a lick anyway (hackers can easily defeat it)... so us honest consumers are really the only ones who are suffering here. I suppose, in truth, that is the most frustrating part.