The Emails are just a notifier, you never trust them to tell you exactly what's going on. And as far as being worried, I was shown how to do a backup once and told, "A few hours in, the backup tape will eject itself. Just push it back in and it will keep going." This struck me as odd, but I was told not to worry about it. When I gained charge, I took a look into it. Sure enough, the backup would overwrite the previous several hours of backup data with the remaining data when the tape was reinserted... What a surprise THAT would have been.
I'm thinking that error has to do with some sort of check on archived files, perhaps through the anti-virus program they use? Could be a corrupt archive. But if you haven't done a test restore on some random files yet, do one now to make sure they've got SOMETHING in case of failure.