It seems I can't rate projects which have Ohloh URL Names. Undoing the URL name assignment makes it work again.
A few days ago I had the very same problem regarding stacking projects, but that seems to work now.
When I click a rating star for Markdown, for instance, Firebug reports:
POST http://www.ohloh.net/projects/5030/ratings404
Instantly followed by:
GET http://www.ohloh.net/projects/markdown/ratings?score=…404
