The latest versions of Firefox also have some really great features for video card hardware acceleration of web pages. Look up how to turn hardware acceleration off in Firefox and that might also help. I’m not positive, but it has a good chance of being your issue if the capture worked before now suddenly does not.
Ping to this post by Advil000. I had a similar problem years ago using a video capture software. It worked fine but anytime I was watching video during a capture I got a black box in place of the playing video. All surrounding items recorded fine.
I don’t recall specifically how I resolved it but it had to with video or hardware acceleration settings so I feel Advil000’s post is heading the right direction. I don’t think its DRM related.