Hi, Tero Niemi,
There have been lots of glitches reported since the iMovie 10.1.15 and 10.1.16 updates. They appear to need Apple to solve them.
You can try the following diagnostic procedures to see if any of them will solve this:
Deleting 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.
Opening iMovie in a new library
Try opening iMovie in a new library. To do that, open iMovie while holding down
the Option key and then selecting to create a new library in the box that
appears.
Opening iMovie in a new user account
Create a new user account from System
Preferences/Users and Groups and try to open and use iMovie in the new account.
You can use the Guest account on your Mac for this purpose.
New Project
Another thing you can try is to copy your
media into a newly created project and see if that cures the problem. To do
that, do an Edit/Select All, Edit/Copy, of the media in your project and then
Edit/Paste it into the newly created project.
Safe Mode
You could also try booting up in Safe
Mode (see Finder menu for instructions) and, after everything loads,
immediately shut down and reboot in normal mode. Then see if iMovie works
properly.
Updating iMovie
To update/redownload iMovie 10, drag your
iMovie 10 app from the Applications folder into the trash, but do not empty the
trash. 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 10. Redownload/update iMovie from there. If not
successful, drag your present iMovie app out of the trash and return it to the Applications folder.
Resetting the System Management Controller
Try resetting the System Management Controller:
https://support.apple.com/en-us/HT201295
Resetting the NVRAM or PRAM
Try resetting the NVRAM or PRAM,
although this issue would be fairly rare:
https://support.apple.com/en-us/HT204063
-- Rich
