With MovieRecorder or MovieRecorder Express 4.1, there is a major "under-the-hood" change. Indeed we stopped using OpenGL for our previews, and are now using Metal instead. Indeed OpenGL si not ideal for Multi-threading, and Metal is now the recommended option by Apple.
For most users this won't have impact (except for some performances optimisation), so most won't see it. But Metal is not available for all computers, and not all Graphics card. You should check the following article to know what is supported:
If you notice that your previews are blank after an update to MovieRecorder (Express) 4.1, this might be the issue. In that case, you should revert back to MovieRecorder 4.0:
We will continue to build 4.0 versions to address some possible bugs for the time being, but we won't be able to do that forever (we might not be able to compile it anymore in the future). So the best is to try to find a Mac compatible with Metal.