WebMethods and IIS7 rewrite fails

I got a WebMethod which I call in a aspx-page. When calling this from a non-rewritten url it is working fine (example: mydomain.com/ThePage.aspx?id=123), but when I call this from a rewritten url in IIS7 (example: mydomain.com/123/Page) I gets this error in Firebug:
401 Unauthorized
{"Message":"There was an error processing the request.","StackTrace":"","ExceptionType":""}

Any ideas what I am doing wrong?
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