Dear Paul,
Yes, this error is notifying you of an incompatibility between the AeroDyn and BeamDyn meshes.
When BeamDyn is enabled, the blade-related inputs and outputs from the ElastoDyn module are unused, including TipRad, replaced with those available in the BeamDyn module.
Your understanding of the aerodynamic center in AeroDyn is correct, but the reference axis in BeamDyn need not be associated with the neutral axis. As has been discussed several times on this forum e.g.: 6x6 Stiffness Matrix of NREL5 MW Turbine - #13 by Jason.Jonkman, I suggest defining the reference axis in BeamDyn as a smooth curve that follows the natural geometry of the blade i.e. likely not the neutral axis, shear center, mass center, etc. Typically this curve can be set with very few key points. Is your reference axis in BeamDyn a smooth curve?
Best regards,