Same garbage happened to me all of a sudden with my 4k Dell UH2414Q.
One week stuff played fine and now for the last week the stupid program just complains about HDCP being missing despite the fact that my system says HDCP is 100% working and every other type of file that has HDCP protection still works fine. Just iTunes refuses to play protected HD iTunes files anymore. What a joke.
HDCP itself is a joke since it does zero to fight piracy. All it does is push to people to have to pirate. What casual user has the fancy equipment required to realtime frame grab an entire HD movie over HDMI/DVI/DP cable?? The big bootleggers have chips to get around it and don't even bother since they hav much simpler ways. So what the heck is HDCP accomplishing other than hurting both the industry and their consumers? It's a total joke.