That's the problem, never had it before. DVD menu backround is way too dark with strong contrast after preparing the project. Bsckround is a jpeg file, in preview window it looks just fine.
When I click optimize dvd, there it says that menu has to be recompressed into mpg file.
Ok, then I tried to do it myself in Vegas, render the jpg into mpg as required, but same problem appears. Something incompatible with the color space of jpeg and mpg files?