Build/Aspect Ratio Error

I've been working for days on my first project, but when it's ready to build I get the message: "The aspect ratio for this file is not supported by DVD". For the first part - which file? There are three video files. The videos have the ratio 720 x 576 (PAL). The menus have the ratio 768 x 576 (standard templates from the Goodies folder).

I hope there's an easy solution here... Any ideas would be great!

Thanks in advance