Integrator error - fixed time step

Hello,

I have got a problem in my simulink model. I had created a model of a mechanical oscillator and when tried to start the simulation an error appeared:

Derivative of block YYY at time XXX is Inf of NaN.  Stopping Simulation.  There may be a singularity in the solution.  If not, try reducing the step size (either by reducing the fixed step size or by tightening the error tolerances.)

I have tried many changes in the model but the solution was following - replace the fixed time step with the variable time step. Than everything worked fine.

Where is the problem? I usually use the time step length 0.001s

The mentioned model is attached. You can find there two blocks. The first one is standard State-space block. The other is the State-space decomposed into fundamental blocks (for comparation).

The fixed time step is set by t_step value defined in InitFcn.