As Perisai mentioned, it seems to be an all-encompassing generic error that has nothing to do with the mileage limit in the case of a oneworld Explorer. In addition to the possible rule violation related to maximum sectors in Asia, other reasons I've found for the error appearing include the tool automatically choosing a flight that departs before the previous arrival and the tool automatically choosing an arrival in one city's airport (e.g. Orly) and a subsequent departure from another (e.g. Charles de Gaulle), thereby adding another sector.