ASP: NET 411 - Content Length Required Error

There are a few questions and answers around the average error 411. But mine is a bit strange: we have a Testserver and a productive one. The error that I describe occurs only on the testerver server, which has exactly the same code as produtive (except for connection strings). We call WebMethod in the code behind:

    [WebMethod]
    public static object GetEnterEditModeObject()
    {
        object btnId = HttpContext.Current.Session[Constants.Session.RoleEditBtnClientId.ToString()];
        var indRole = (V_WFA_ROLES)HttpContext.Current.Session[Constants.Session.RoleCurrentDatarow.ToString()];

        if (btnId != null && indRole != null)
        {
      ///... Some more code here

            var result = new
                {
                    btnId,
                    ///More Variables here

                };

            HttpContext.Current.Session[Constants.Session.RoleCurrentDatarow.ToString()] = null;
            HttpContext.Current.Session[Constants.Session.RoleEditBtnClientId.ToString()] = null;

            return result;
        }

        return null;
    }

Nothing special here: we read two values ​​from the session, if there are any, we create a new object, clear the sessions and return the object, otherwise we just return null.

This worked fine for a long time, but now we get the error that I mentioned above. Since he is still working on production, I can easily hide it, and I get these two headers:

HTTP/1.1 411 Length Required
Date: Mon, 24 Feb 2014 13:10:40 GMT
Server: Apache
Content-Length: 344
X-FRAME-OPTIONS: SAMEORIGIN
Connection: close
Content-Type: text/html; charset=us-ascii


HTTP/1.1 200 OK
Cache-Control: no-cache
Pragma: no-cache
Content-Type: application/json; charset=utf-8
Expires: -1
Server: Microsoft-IIS/7.5
X-UA-Compatible: IE=9; IE=8
Date: Mon, 24 Feb 2014 13:12:22 GMT
Content-Length: 10
Via: SomeURL

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HTTP/1.1 303 See Other
Date: Mon, 24 Feb 2014 17:04:33 GMT
Server: Apache
Content-Length: 196
Location: /error_path/400.html?al_req_id=Uwt7oX8AAAEAAGijHI4AAAYG
X-FRAME-OPTIONS: SAMEORIGIN
Keep-Alive: timeout=10, max=500
Connection: Keep-Alive
Content-Type: text/html

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