After burning almost 20 (!) sample videos I finally figured it out. I had to match these four settings:
camera setting: widescreen
sony vegas properties/template: PAL dv widescreen
sony vegas render template: dvd architect PAL widescreen
sony DVD architect 8: PAL widescreen

If one of these is set to PAL instead of widescreen the interlace lines appear.

Problem solved, thank you for your reply's
Rene