Hi Emefun,
A couple of things to fix:
(1) add mass blocks. The "dynamic variable derivatives" error is most likely referring to applying a force to where the mass is zero, so inf acceleration. When you had a linear spring, the solver may be able to simplify the equation enough to avoid this, but it's not in general possible for nonlinear spring.
(2) I noticed you've specified nominal values in all the nonlinear spring blocks. I suspect that you meant to set initial conditions. It should be done in the "Initial Target" section. It's very rare when it's necessary to modify the nominal values.
Making these two changes and the model will run. But it runs very slow. I looked at the results and I believe it's due to the very high frequency of the oscillation you have in the system. Please check whether the parameters (spring constants, masses, etc) are correctly set, including the units.
See attached model.
