This usually means that something is wrong with the video file or there is an issue with your DVD disk. To ensure it is not a problem with the video file, please use RealConverter and convert the video to MP4 (even though it is already in a MP4 format). This will ensure the video is designed to work with RealPlayer's DVD burn function.
Second thing to look for is your DVD disk. Try using a different DVD disk, preferably a different brand of disk.