I think you are correct with saying that it isn't a proper mailbox. But still, I would like to be able to catch that error instead of this.
Could you create a support call? Post the support case ID here and the follow-up after that
By the way, easiest way to exclude it (for now, it will be easier in v1.5) is to create a new job, with that specific mailbox in it, and then disable the job. For your other job, you can still choose to select ALL mailboxes but since the specific one is mentioned in another job, it won't be in there anymore. But as said, a better solution in 1.5