upgrading on old MACOS

Rich mentioned that after temporarily dragging iMovie to the trash going into your Purchased page in the App Store.

Did you actually access your Purchased page in the App Store by clicking on the Purchased tag at the top of the App Store as in this picture?

With the iMovie App in the trash do you see iMovie listed on the Purchased page?

If you do, what does it say against it? Update, Download?

Updating/ downloading from the Purchased page will give you a message that you cannot get the latest version BUT it will offer you a version that is compatible with you current OS .

I believe you have tried this before but, worth trying again.

Assuming that you are trying to update an earlier iMovie 10 version, try this procedure:

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.

-- Rich

Definitely upgrades can cause conflicts on your system.

Your iMovie version is pretty old, for sure. And indeed that could cause the crashing, as you suspected. Let's see what happens after you run the diagnostics. It should take about 10 minutes.

-- Rich

Hang in there.

— Rich

That is correct.

-- Rich

O.K. Your setup is not allowing an update to a newer iMovie version, so we move on to something else. Actually, maybe using your Time Machine backup to restore an earlier rather than later version might work. Then again, the crashing may have nothing to do with the version. The version recommended for High Sierra is iMovie 10.1.9. Since your computer can run High Sierra then it should be able to run iMovie 10.1.9. You didn't mention your computer specs or what version of iMovie 10 you are presently running.

Crashing can be caused by a lot of different things: insufficient free app space on your drive; corruption of the iMovie library or a project in it; a conflict in your user account; a conflict in the system somewhere. First confirm that you have sufficient free space for iMovie to operate. If all O.K. there then . . .

Try running through some diagnostics.

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.  Deleting preferences is a safe procedure that will not cause data loss or disruption to your project.

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.

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.

Resetting the System Management Controller

Try resetting the System Management Controller (but no longer available for the new M1 computers):

https://support.apple.com/en-us/HT201295

Resetting the NVRAM or PRAM

Try resetting the NVRAM or PRAM:

https://support.apple.com/en-us/HT204063

Contact Apple Support

If none of the above resolve your issue, contact Apple Support.  The techs there have the capability of remotely accessing your computer that we cannot do here.  To contact apple support click on the Get Support item in the upper right hand corner of this forum’s screen.  Navigate to a place where you can enter your phone number.

-- Rich