mixdown problems cubase artist 7

today I have a problem with audio mixdown with cubase artist 7: when carrying out the mixdown, cubase crashes.
This is happening with all sessions I played in any day before today.
On the contrary, if I play a brand new session today, mixdown works.
I never had this problem before.
I haven’t done any update on windows 8 recently.
My cubase version is: 7.0.4.
Thanks for your help.