While odd, I think this is a "Mavericks doesn't play well with Encore CS5.1" issue.
I "thought" from other posts that EN 5.1 Mavericks users got it to work okay. If so, try the Mavericks fixes.
If not, no idea.
My belief, based on seeing fixes posted for various things on the After Effects forum, is the Mavericks breaks folder permissions. But I really don't know.