Matlab crashes when SImulink is sorting block list for propagation

Matlab crashes when Simulink just it starts compiling after: it says at the bottom of the Simulink diagram: Compiling: Sorting block list for propagation

How can a Simulink compile crash Matlab???

The same model supports a number of use cases, 50% work perfectly 50% crash Matlab altogether, no error numbers, nothing. The model uses the discrete solver with a fixed step and it is rather complex. Matlab is version 2015a.