After your post I have tried the same, and this happens in my system too. Black video, but sound is OK. Mind you I apply the effect to the whole video.
When I move the video with the effect to the first line, the black remains. After I have removed the effect, the video returns, but PD keeps rendering a couple of seconds from the cursor. PD keeps doing this even when I have removed the video and replaced the same. This is over when I restart PD and move the (same) video to the timeline. Then it plays (and shows) without rendering.
When I move the effect to the effect line right under the first timeline, with the video in lane 1, the effect of the effect is shown as expected, but rendering happens. Even after I have removed the effect from the effect line. And even when I have removed and replaced the video in the timeline without effect: rendering...
Something we may want to pass on to Cyberlink, when more of you see the same thing.