Yes if a new user works OK - then the problem use to be one or more corrupted pref files.
and they are hidden in the Library IN YOUR USER ACCOUNT FOLDER the other one on first window on your Mac hard is is the Root Library - the wrong one.
In Your User-Library / Preferenses You find some named
or resembling this. Search forum and You get the exact names on them
move them to destop - now start iMovie again and new fresh ones are created
Yours Bengt W (not at his Mac but on an iPad - so I can not see the proper names)