That error is probably due to a change in how quaternions are handled in Rotations.jl. I don’t know the history of it, but there’s an open issue and a pending PR. It appears that things got switched from UnitQuaternion to QuatRotation, and field access to w, x, y, z was not retained. The PR should restore this with appropriate getproperty, but RigidBodyDynamics could also avoid field access altogether, since it is brittle.
You can try out the PR and see if it fixes everything for you. I don’t think this has anything to do floating vs fixed base.