I don't think unchecking "Optimize Stills" will change anything, at least for me it dosen't.
Looking more into this, It seems to be more of a memory problem. For example, I had NO problems editing, rendering, or exporting the beta of my new video which uses old footage that is not currently filtered. But when I did the same for my newer and cleaned up Gunslinger Girl segment for a MEP, it died three times while trying to render. I seem to always have this problem with newer anime like GG, Madlax, or Kaleido Star.
If that helps with the troubleshooting going on.