I've also had this issue when generating optimized media. In my case, it works for small batches (up to about 15 clips). But for 20+ clips it would always crash randomly. I had to just generate the media in small groups.
In my case the error is totally random, it could happen with one clip or with a small buch. It usually happen within 10 minutes of computational activity.
Christian, please don't follow all that guy says! The workflow is basically alright, but it's complete BS to transcode a GOP format to another GOP format for proxies. And it's not really complicated.
Use Cineform at half rez or DNxHD LQ instead (or ProRes LT an a Mac).
Well, I think I'll have to go another route then, since this morning I've tried transcoding some clips, as indicated by the youtube tutorial, and after a while I got the same message I get with optimized media generation (the frame such and such as been dropped) and then crash.
Also, the transcoding wasn't using my GPU (amd radeon 5700) but only the CPU. It crashed after about 20 minutes of work.