Ugh. I hate Quicktime!
Movie Maker can be finicky but is jam packed with bugs hidden features that keep you on your toes. It may not work very well, but it is slow!
Do you happen to know if you are using an account with administrative privileges? Sounds like it could be a permissions problem perhaps if you have enough disk space. Maybe try running the program as administrator (Right click the program icon, and left click “Run as administrator”) and try again. I try and only feed Movie Maker .mp4 and .mp3 files, and ask it to spit out .mp4 files. All else fails, create a new project and start over. Movie Maker likes to corrupt project files. Before you go spending a lot of time on an edit, try making a single change first, then saving.
I haven’t found any good free editors that I prefer over Movie Maker, so I learned to work around the bugs when I have to use it.
For pay options I prefer Final Cut Pro on Mac, and Cyberlink PowerDirector on PC.