Having the same problem.
Upgraded to Mavericks and iPhoto 9.5, and now getting this, every single time.
Happens with both Original and Current exports. The failed files are all point to the Previews location. I checked the first few, which do exist, but are 0KB big. However their master file location is fine.
This ends up leaving 120GB out of my 170GB collection of 1533 photos/videos.
Incredibly frustrating! iPhoto is useless to me now, and I have no way to get my photos out of it.
Here's the exact feedback I sent apple:
I'm experiencing this issue:
https://discussions.apple.com/thread/5478191?tstart=0
Since upgrading to Mavericks and getting the latest iPhoto version. I cannot export my entire photo collection (using original, and current settings).
It always fails with about 120GB of my 170GB collection left. The missing files are videos that iPhoto is trying to copy over the preview location, rather than the master location it seems. The discussion thread has more information.