Got this nice bug coming right out of the Zend frameworks 1.9 AMF Routines.

“Unable to parse null body data flex.messaging.io.ArrayCollection mapped class  is not defined”
in file: library/Zend/Amf/Request.php:174

Was trying to send an object that has an Arraycollection as one of its parameters to the gateway.
Found this Bug Report http://framework.zend.com/issues/browse/ZF-6601
It is supposed to be fixed in 1.8.2 ?! Why is it happening in 1.9 then?

Instead of modifying the zend framework I decided to convert the object I wanted to send into an object without arraycollections.
But definitely not a proper solution. Hopefully that bug will be fixed soon. Otherwise maybe AMFPHP is a better choice then Zend AMF.