Hmm. I've had this problem with one of my Faces. Is there a space in the file name? If so, follow this:
1. Rename the face to have no spaces.
2. Put it back in the .zip.
3. Test again.
4. If it doesn't work, try removing some faces from the Add-Ons folder. Then do 3 again.
5. ????
6. Profit!