Dear Ryan,
If you are confident that the grid is large enough to encompass the rotor under deflection, than the only problem I can think of is numerical instability leading to large deflection. Are you getting the “small angle approximation violation” warning?
You mention that you are modeling an idling rotor, but are receiving an error from AeroDyn v14 subroutine infdist(). However, this subroutine is only used by the generalized-dynamic wake (GDW) model of AeroDyn v14 (DYNIN option). For an idling rotor, it is normally recommend to disable all wake calculations, i.e. set IndModel = NONE, which will disable GDW. Perhaps GDW is driving the model unstable for your case.
Best regards,