Why does (-50) come up as a problem for me when i try to publish?

when i try to publish my iMovie to iTunes, it keeps coming up with...

Unable to prepare project for publishing.

this project could not be prepared for publishing because an error occured. (-50) 😠😠😟😕😠😠

does anyone know what that means, or what i can do to export it. i have made movies in the past, and they were about 1:30 to 2 minutes long before, and now suddenly it won't let me export it. i have music on it, and the movie is about 4:30 long. it has video and fast forward and slow motion and rewind. but that shouldn't be a problem because movies are not exactly the shortest things.😕 if anyone knows about this or knows how to fix it, could you please tell me because it is so annoying watching that sign come up every time i try to export it. so thank you for your time.

Millstruck 😎