Hi Perrin,
I noticed this was happening when I used:
target-global-mbox
as the mbox, rather than providing a more specific custom mbox. Once I provided my own custom mbox, the error was no longer there.
I don't know many details, but I heard this may be a bug within at.js. I don't know the extend of the bug, but it was mentioned in a forum. Hoping someone can chime in at some point to confirm.