I'm responding to my own request here because I figured out the problem and I thought that it might be helpful for someone else.
The problem was one of the video files. I had created a separate slide show video (with powerdirector) as an mkv file and added it to the timeline at the end. After adding it, I redid the file, removing, some pictures, and saved it again. PowerDirector immediately used the new file and I wondered if that was the problem. So I removed it and was able to successfully burn the DVD. But I wanted the slideshow included so I tried adding it back in and the same problem occurred. So instead of adding the mkv file I imported the PowerDirector slideshow project into the main DVD project and it worked fine.
Hope this helps someone.