If the video gets stuck while you play it in iMovie, then perhaps remove the clip/photo causing the issue, it may be corrupt.
If the movie is playing fine in iMovie, there is no issue with the individual photos/videos, the problem may be with the project file (*.imoviemobile file). Which is causing it not to complete the export and gives an error.
Try this:
- plug phone in to PC using cable and open iTunes.
- open iMovie app on phone, press on projects, then your movie, press the square with arrow UP icon, press itunes, press iMovie Project. This will transfer your project file to iTunes.
- go in to your connected phone in iTunes on PC, on the left click Apps. You will see your phones pages. scroll all the way down to the File Sharing section, under apps click iMovie, you will see your file there. Save it to desktop, rename the file, then add it back in. Now you will have 2 files, original file and renamed file.
- go to iMovie app in phone, press on projects, press square with arrow DOWN icon, press iTunes on bottom and import the NEW filename into iMovie app.
- export the new file to camera roll