Hi,
Error 500 makes me think that the request was issued by TestComplete but caused error on the web server.
My suggestion will be to use some web debugger (Fiddler or WireShark, for example) and record request sent by TestComplete to the server.
Having recorded request, you may examine it and: a) either find what is incorrect in the request by yourself and then decide how the test code should be corrected, or b) talk to developers and ask them to consult you on what might be wrong with the given request.
The good idea may be also to talk to web server admin and ask him to check web server logs (and/or system event logs) which might contain additional information from web server/web service.
