Widows 10 Movie Maker: supports mp4 but sometimes works, sometimes doesn’t

I've read that mp4 is no longer supported on Win 10 Movie Maker.

Sometimes I can import mp4 from my video camera, sometimes it errors out.

Does anyone know for sure, if it works or not on Win 10? Its very frustrating when one day it works, one day it doesn't.

I've reinstalled Movie Maker with no success.

If it doesn't support, can you recommend a good, free alternative?

Thanks for any advice!