What OS? What exact version number of After Effects? What is the exact message that you're seeing?
I'm getting this, too!~
I was working on this on a different computer, brought my files home and now this! This project will never end...
AE CC - 2014.0.2
I think I figured out my problem. It was a folder permissions issue. Every time I take my drives back and forth between these two computers, I have to reset the folder read/write permissions...
How it was that easy.