Also try 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.
If still no luck, try these other diagnostic procedures:
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.
If none of the above cure it, try importing your HEVC clip into QuickTimePlayer and then exporting it out to clear it of HEVC. When you export there may be a box to uncheck. Then see if it works in iMovie.
-- Rich
