Q: Can't open iMovie library.

After upgrading from El Capitan to Sierra, iMovie wont open my library.

I get this message:

The operation couldn’t be completed. Permission denied

“iMovie Library” is already in use by “my name” on “another name”.

I have read and write permissions on the library but I notice that I am not the owner and the option to make me the owner is greyed out.

Any ideas on how to fix this?