I took a quick look for a similar thread and didn't find anything, so apologies if I missed it.
I've had this happen a couple of times: when I change the primary SMTP address for a user, my job fails. My job is currently set to backup all mailboxes, so I wouldn't expect it to fail. It appears that it is cleared up by going through the job setup process again. Interestingly enough, new mailboxes are added without issue, but changes to existing mailboxes appear to cause problems.