I believe the whole flight plan, including previously passed waypoints, is 100 legs maximum. As I understand the “fix” is to either: (1) drop back to heading mode, do a direct-to using the DTO history to a waypoint behind you, and delete passed waypoints to make room for more legs at the end of the flight plan; or (2) use the secondary flight plan to construct a new, shorter route and then swap it into the active flight plan.