It just happened to me. A friend told me a solution, and it worked. In this case, the first frame of the clip was corrupt and that is why it would’t play. The solution is import the mp4 to FCP and make a new sequence. Drag the file to it. FCP will say “general error” but you will be able to continue. Cut the first frame, again it will say “general error”. Render the clip from the timeline and it will work well. It happens to me with Videoblocks mp4 clips
Monica Peña