ASP.NET, The status code returned from the server was: 500
<p>A customer using a ASP.NET program I have just sent them is experiencing the error below, I had personally tested the program on 4 web server IIS6, 7, and 7.5, on 4 different networks and I never got this, the customer is using IIS6 with .Net 2.0 (as required). The program uses Windows authentication which may or may not be part of the problem. I'm stuck I don't know where to start with this. Any help is very much appreciated.</p>

<blockquote>
  <p>Sys.WebForms.PageRequestManagerServerErrorException:
  An unknown error occurred while
  processing the request on the server.
  The status code returned from the
  server was: 500</p>
</blockquote>
