Hi!
Which plug-ins are you using? Check the clip at 50-60% on your timeline. I had two identical filters on one clip, and received the Unknown Error.
Reduce your effects, they write big temp files.
And the last method, which takes a little time but seriously saves lifes (we have premiere on sunday): Export in parts. 6 minutes is not that much, but who knows. Take a good codec (I’d personally go for H.264, Uncompressed Microsoft Avi makes my media darker), re-import, export in same codec. I got no frames missing.
Hope I’ve been of some help!