https://marco.org/2008/10/12/the-imovie-08-error-when-you-try-to-export-too
The iMovie ‘08 error when you try to export too much AVCHD footage at once. Google thinks this means “file too big”. Is there a 32-bit integer somewhere where a 64-bit should have been used?
The best part is that this error only happens after the entire conversion, right at the end, after hours of processing, and then it deletes the unfinished file.