by Matt Gray.
Having the same issue - our moodle server died, so we set up a new moodle and restored the course backups, and about half of them refused to restore with this error. After initially panicking that the backups were bad, we tried restoring them it to another VLE we have running version 3.9, and they were fine. Ultimately, deleted moodle and reinstalled 3.9, so we could restore the backups. It seems to be an issue with 3.11.