by Iván Yivoff.
Ken, thanks for your help, but I did say which command I'm using: moosh course-restore. And I don't know of any other command useful to restore a backup.
It always behave the same way: creates an empty course.
Not about memory or timeout, and not dependant on a particular server (happens both on the production machine, dev machine, local dev machine, etc) it is clearly a php exception.