cURL gets Internal Server Error when posting to aspx page

I have a big problem. I have some applications made on an unix based system, and I use PHP with cURL to post an XML question to an IIS server with asp.net. Every time I ask the server something I get error:

HTTP/1.1 500 Internal Server Error
Date: Tue, 04 May开发者_运维知识库 2010 07:36:08 GMT
Server: Microsoft-IIS/6.0
X-Powered-By: ASP.NET
X-AspNet-Version: 2.0.50727
Cache-Control: private
Content-Type: text/html; charset=utf-8
Content-Length: 3032

But if I ask same question on another server, almost identically to this one (BOTH configured by me) I get results like it should and the headers:

HTTP/1.1 200 OK
Date: Tue, 04 May 2010 07:39:37 GMT
Server: Microsoft-IIS/6.0
X-Powered-By: ASP.NET
X-AspNet-Version: 2.0.50727
Cache-Control: private
Content-Type: text/html; charset=utf-8
Content-Length: 9169

I tried everything, searched hundreds of forums, but i don't find anything.

In IIS logs I only get:

2010-05-04 07:36:08 W3SVC1657587027 80.xx.xx.xx POST /XML_SERV/XmlAPI.aspx - 80 - 80.xx.xx.xx Mozilla/4.0+(compatible;+MSIE+6.0;+Windows+NT+5.1 500 0 0

any ideas where to look what is going on?

I forgot to mention! If I use an XML request software, and ask same question, it works.

Try reducing your asp page to the minimum, with the first try with an empty page. If this succeed, begin to add the real bits until it fails, so you can narrow the error.