CST : Frequency Solver Error

Dear all,


I'm trying to do some simulations and sometimes , when i use the Frequecy domain solver, after the mesh an error appears to me and also a pop up asking me this :

- do you want to start again with an alternative port mode solver?

If I press on the OK button, the simulation restarts without any issue now.
Sombody could explain me what is happening? Can i change somewhere this option?

Thanks in advance,
V.M.