The 500 your getting is probably based on one of two things:
- An incorrect request
- An error on the server
Now, if its an incorrect, the HTTP server would be more helpful responding back with like a 415 (unsupported media type) or something. A 500 insists that something went wrong on the server and that your request was valid.
You'll have to dig into the server API or code (if you wrote it), or read the docs and figure out what's wrong with your second request ... seems like maybe your not setting the appropriate media type?
EDIT: Ok, so I looked at the API. It appears your posting XML, so your request content-type should be
Content-Type: application/xml
The API doc didn't specifically say, but that would be the correct type.
EDIT: Actually on second glance, are you just POSTing w/URI params? Their API doc isn't clear (I'm also looking rather quickly)