I was having this problem too.
I had a video in my presentation where I had used the 'Trim Video' function to shorten the video.
I deleted this video from the presentation, then trimmed it to the required length in a separate program (in my case, I used Windows Movie Maker).
Then I reinserted the new shorter version of the video into the presentation.
Powerpoint's export to video was then able to complete successfully.




Hopefully this is a solution for you!