public static String SOAP_ACTION = "http://10.0.2.2:28266/Service1.asmx/abmeldung";
I think your soap action should be something like,
"http://tempuri.org/Service1/abmeldung"
You need to check this in your wsdl.
Open your wsdl in browser using,
http://10.0.2.2:28266/Service1.asmx
Search wsdl:binding tag in wsdl.
Within binding tag search for operation ambeldung
You can find your SOAP Action defined as,
<wsdl:binding name="BasicHttpBinding_Service1" type="tns:example">
<soap:binding transport="http://schemas.xmlsoap.org/soap/http"/>
<wsdl:operation name="ambeldung">
<soap:operation soapAction="http://tempuri.org/examplesoapaction/example" style="document"/>
......
......