Oops, I gave the web server a kick and the 500 errors are gone.
I'm afraid the documentation sucks. Really, the client.py code is the best, along with the server-side urls.py (
http://trac.astrometry.net/browser/trunk/src/astrometry/net/urls.py). If you search this forum for "api", you'll see a few threads that cover some API issues (sometimes in gory detail).
cheers,
dustin