It seemed to be struggling earlier, perhaps it has hit some sort of bandwidth quota and the error response is an unhelpful 403.

I've seen that before, perhaps it counts as unauthorised because the server is not authorised to send when the quota is breached?. It isn't correct use of 403 (which implies the server knows the identity of the user, 401 is the more general state for unauthorised. Of the standard responses 503 (Service Unavailable) seems most correct.