I got another backup from the site, and tried to restore it via terminal using this howto: Restore a backup from command line. but then at the end of the restore, I got the same error in the terminal:
Notifying 'system' of the end of the restore...
Something went wrong while notifying user.
Finished!
[FAILED]
Restore done.
since the backups were fine, I guess this is a bug.