it may happened due to a infinite value (dividing by zero) enters to integrator block.
1- consider an initial value of integrator.
2- consider upper and lower bound for integrator
3- you could also change the minimum step size (in configuration) but, it may not be solved, because the solver would try to reduce the step size to deal with big numbers and then this may happened again!
4- change the solver type
5- limit and consider initial conditions for all integrators and functions
You know, a adaptive step size integration method (ode45) is used in Simulink. If the calculated tolerance is larger than the threshold, the step size is redused. When the step is smaller than the specified minimum step size, this warning may occur. You can reduced the minimum step size and see what happens. This problem is usually because the highly nonlinear dynamic model. Please chek the model first.