What is QuickTime "unknown error (-50)"?

I'm converting MKV files to MP4 with ffmpeg, and when I try to open some (but not all!) of the resulting files, QuickTime Player (version 10.4 on macOS 10.13.6) produces an error dialog with the message: > The document “….mp4” could not be opened. An unknown error occurred (-50) What is this "unknown error (-50)" that is preventing my MP4's from being opened? How can I get more details about the error so that I know how to fix it?