Asynchronous operations are not allowed in this context.

Hi,
I tried to execute my code async but get this error:
Asynchronous operations are not allowed in this context. Page starting an asynchronous operation has to have the Async attribute set to true and an asynchronous operation can only be started on a page prior to PreRenderComplete event.
On the net I find it is bound to Async="true" missing in page declaration but this is not the case here. So what else could be causing this behaviour?
<%@ Page Title="" Language="VB" Async="true" validateRequest="false" MasterPageFile="admin.master" Debug="true" AutoEventWireup="false" CodeFile="eifeler-regel-letzebuergesch-checker.aspx.vb" Inherits="eifeler_regel_letzebuergesch_checker" %>
<form name="aufnahme2" id="aufnahme2" method="post"> <p class="pos_fixed"><input id="gre" type="button" name="Richt ...