And I'm back with the same error, this time doing a backup/restore with users in Moodle v2.6 back to the same server.
Unfortunately, I don't think we ever resolved the issue I reported previously (backup in Moodle v2.6, restore to Moodle v2.8).
However, based on your query, I re-tried the backup/restore after removing the Quickmail block from the original course. The course successfully restored. Thank-you!