It seems that thanks to your help, I have finally figured it out! I now have a working DVD (23.976 fps) and everything seems to be fine. After taking all the necessary steps one by one, I even found a program that can do the frame conversion, the resizing and the muxing all in one. It needed ten hours on my old computer, but it worked.

But both methods have one last problem in common: The audio is not in sync with the video. I'm ashamed to say that I know next to nothing about dvd authoring ... can you recommend a program that can solve my problem? I know I have to change the delay of the audio file, but how?

Edit: It seems that I have found a solution for the problem ...