What is reason for Xpc target average TET = None?

When I run my model in Simulink with fixed time step it works fine, but when I run it in Xpc, the Average TET says None and after a while I loose contact with the realtime computer.

I have run parts of the model in Xpc and Average TET is then ~1e-5 but when I run the full model the Avarage TET becomes "None".

Does anyone know the reson for getting TET = None?

Thanks //Marcus