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 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.