Error 50 is a rendering error that indicates that there is some corruption in the project or one of the clips in it. Try opening your iMovie project and then expand your timeline full out using the slider next to the settings button above the timeline to the right of your screen. Then put your cursor at the beginning of the movie in the timeline and slowly scroll through the timeline. You can use the left and right arrows at the lower right hand corner of your keyboard for this purpose. Look for white flashes, black frames, artifacts, and any other signs of corruption. Cut out any that you find and then try again to export.
You might want first to duplicate your project and work with the duplicate if you want to preserve your original project. You can make a duplicate by opening the project browser screen (the one that displays your projects as icons) and then move your cursor over the project's title. A little circle with dots in it will appear next to the title. Click on the circle and you will get a drop down menu that gives you a Duplicate Project option. You can make as many duplicates as you want.
Other ways of tracking down corruption in a project is to work with duplicates and export half the project to see if you get a successful export. If you do then you know that the corruption is in the other half. Then go back to another duplicate and export half of the other half that contains the corruption. Repeat until you track it down.
-- Rich
