Don’t know if anyone else is getting this, but I can’t export my models from 2.121 regardless of format, however 2.120 works fine.  They are running on different machines, but I don’t see how that would affect it…

There is no diff with the 2 versions in that regard. (works fine here on both mac and PC)
Make sure your privileges/rights/schmod is set correctly in your destination directory, as Prefab must have write access. Try for instance to export to the public desktop.
Also a known common failure is trying to export into svn controled directories.

It is on my todo to implement some warning for this. (actually there is already one but fails if read access is off)