Thanks for the hint!
That explains a little but doesn´t seem to solve my problem. I used the
charles debugging proxy (http://www.charlesproxy.com/)
to check whether there is an amf request dispatched at all but nothing.
What strikes my odd is that in firebug I discovered that there is a
request to http://localhost/crossdomain.xml with 404 Not Found of course.
Could that be the problem in the first place?
I found the answer myself.
I had a look at the files generated out of the .fml file, i.e. _Super_MyApp.as and there it said under _serviceControl.endpoint "http://localhost/and so on" where in default configurations there would be "gateway.php" only. As I integrated the gateway.php into my Zend Controllers .i.e. AmfController.php I had to put <item name="Endpoint">../amf</item> in my .fml file. Now it seems to work.
However I still wonder what this does...
... urls and paths are obviously wrong here but that doesn´t seem to be a problem when exported. I guess this is only for introspection of the services in flashbuilder, right? Or am I going to run into more problems?