If I recall correctly, 500 is the prescribed status code for JSON-RPC error responses. There is still a JSON response in the body of the reply telling the explanation of the error, which could be something like {“result”:"",“error”:“bitcoin address not found”,“id”:“1”}.
Re: bitcoind not responding to RPC
Figures: Satoshi Nakamoto
Satoshi Nakamoto July 23, 2010 17:23 UTC