Two DoF Non linear mass spring damper system with lookup tables, help with ODE

Hi everybody, that's my first time writing here, so I apologize in advance for any mistake.

I'm trying to integrate the mathematical model of a landing gear drop test, modeled as a two dof mass spring damper system. The system and the two free body diagram are the following:

I wrote the system as 4 eq. one order, using some lookup table to solve the non-linearities:

      function [ yp ] = sistema_interpolato( t, y, D, z_s, z_inf, m1, m2, delta_ref, F_el_sa_ref, deltadot_ref, F_damp_sa_ref,delta_tyre_ref, F_el_tyre_ref)

      x1 = y(1);
      x2 = y(2);
      x1dot = y(3);
      x2dot = y(4);

      delta_sa = z_inf - (x1-x2); % with z_inf that's the 0-force distnace between x1 and x2

      F_el_sa = interp1(delta_ref,F_el_sa_ref,delta_sa);

      deltadot_sa = x2dot - x1dot; % compression/extension speed 

      F_damp_sa = interp1(deltadot_ref, F_damp_sa_ref, deltadot_sa); 

      delta_tyre = D/2 - x2; % tyre compression, with D = tyre diameter

      F_el_tyre = interp1(delta_tyre_ref, F_el_tyre_ref, delta_tyre);

      yp(1,1) = x1dot; % x1dot
      yp(2,1) = x2dot; % x2dot
      yp(3,1) = (1/m1) * (-m1*9.81 + F_el_sa + F_damp_sa); % x1dotdot
      yp(4,1) = (1/m2) * (- F_el_sa - F_damp_sa + F_el_tyre); % x2dotdot

then I try to integrate with all ODEs solver in matlab and with an Euler fwd method implemented by me, obtaining always this warning:

Warning: Failure at t=3.909016e-02. Unable to meet integration tolerances without reducing the step size below the smallest value allowed (1.110223e-16) at time t.

with t variable, depending on the time interval and the initial conditions (5,5,-3.7,-3.7).

Does anyone have any idea?

For this project I cannot use simulink unfortunately.

Thank you very much in advance :)

Grazie Mille!

[Mischa Kim]

Paolo, please attach a copy of the derived differential equations and the entire code. My first guess would be a sign error in your equations that result in an unstable system.