Before going any further, try deleting preferences since it is so easy to do. 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. Now see if you can share out.
If still no luck, there's some corruption in there somewhere. Sometimes it is hard to find.
Try this. Make a duplicate of your movie and work with the duplicate. Split off the last half of your movie and delete it. Then share out the first half. If it shares out, you know that the corruption is in the last half. Make another duplicate of your original movie and split off and share out half of the last half. Repeat until you find the corruption.
On the other hand, if the first half does not share out, try making a duplicate of your original movie and sharing out the last half to see if there is corruption in it. Keep repeating the share process until you can narrow down where the corruption is. It could be in a title.
Sometimes titles become corrupted, particularly third party titles. Go to Utilities/FontBook.app and select all the fonts that are displayed, and then click on File/Validate Fonts. Disable any fonts that are in your movie that display problems.
-- Rich