ODE Solver - errors with differential equation

Hey,

I am trying to model the second order differential equations for the shock absorber. I would like to get the funtion of the movement of the mass. I am getting errors from the Solve Block in MathCAD, and I cannot understand where is it coming from. I suppose the reason is the sin function x(t), but I have no idea how can I write it differently. Without units the solver was working fine, but with units I get always an error. I am uploading the MathCAD file.

I would be glad for some hints and tipps! Any help is welcome!  

Best regards,

Jolanta