Hi, I was experiencing the same problems as you are with Windows 7 Premium. Are you using Wavelab as a stand alone program, or are you launching Wavelab as an external editor from within Cubase?
I was able to get my Wavelab Essential to work properly after assigning it as the external editor and launching Wavelab from within Cubase. No more crashes or closing of Wavelab for me as long as I don’t use it as a stand alone program. 