QuickTime Player won't save edited file: The operation could not be completed

I've edited an MP4 movie file in Quicktime Player Version 10.5. When I try to save the edited file, I get: The operation could not be completed. Does anyone know what causes this and/or how it can be solved? macOS Mojave 10.14.2 / Quicktime Player 10.5 / MacBook Pro (Retina, 13-inch, Early 2015)