Hi,
Try booting up in Safe Mode (see Finder menu for instructions) and, after
everything loads, immediately boot up in normal mode. See if that cures it.
Also, you can try opening iMovie while holding down the Option and Command
keys and selecting to delete preferences in the box that appears. iMovie will
open in a new empty library. Reopen your old library to get back to your
projects. Now see if you can share out properly.
If neither of the above procedures solve it, try redownloading iMovie. To do
that, drag your iMovie app from the Applications folder into the trash, but do
not empty the trash. Then sign in to the app store and open your purchases
folder by clicking on your name at the bottom of the app store side bar. You
will see your purchases displayed, including iMovie. Redownload/update iMovie
from there. If not successful, drag your present iMovie app out of the trash
and return it to the Applications folder.
-- Rich