This was my project, I zoomed in on all the clips and manually skimmed through them by moving the mouse over the clip. Doing that shows you a quick preview of the output.
I noticed that one of the clips that I imported flickered when I was looking at the preview, so I narrowed down where the flickering was occurring and got this black frame. That was the culprit.
Just before this black frame I clicked the video to put down a marker and split the clip (Command-B).
Then I found the part of the clip that was after the black frame and put another marker down and split the clip again. I then deleted the clip with the black frame in it. I probably lost a few seconds of video then but since my footage was sped up 15x anyway, it didn't matter.
After doing this I was able to export the movie without the error popping up!