Error importing imovie mobile to fcpx

I have a student's project started on iMovie for iOS that I'm trying to import to FCPx on my Macbook for finishing. When I try to import I get the error message "Failed to parse project metadata and information. File may be corrupted or have the wrong extension."

Things I've checked:

All OSs and apps are the latest versions.

I can open and play the project with no problem on the iPad.

When I Airdrop the project to my Macbook the file transfer completes successfully.

Backups of an early version of the project import with no problem - it's only the most recent version that is causing trouble.

There's no difference in the effect whether I open FCPx and use the Import menu, or choose "Open with...." in Finder.

Can anybody help? Recreating the whole project from scratch is a last resort I'd like to avoid.