What programs are you running? This can happen if, for instance, a VNC session is started, or one of the many airplay applications (servers) runs, or if a screen capture utility runs. They create a third virtual monitor which isn't HDPC compliant, and if they happen to do it while you're watching your movie, you'll receive this error.
Check through your services and background apps to see what is running that might need or use display access. If you don't notice anything, see if you can disable all your background apps and test it again. If it's still happening you might need to reinstall OS X to get rid of any old drivers left behind from an installation of an old USB display adapter, for instance, or screen sharing application.