The export malfunction could be related to just this one project. To find out, do and Edit/Select All, Edit/Copy, of the collective media in your project, and the Edit/Paste it into the timeline of a newly created project. Your edits will carry over. Now see if you can export a workable movie without issue.
If it is not a project related issue, it could be simply a corrupt iMovie preference, in which case you can do a delete/reset preferences procedure that is harmless and will not result in any damage or lose of date in your project. To delete preferences, open iMovie while holding down the Option and Command keys and select to delete preferences in the box that appears. iMovie will open in a new library. Reopen your old library to get back to your projects.
— Rich