Hi Florian
Sorry for the delay in response.
From your error message, it looks like there is no matching sender agreeement for the response of the sync message. Based on William's blog, this should be item #4 in the list. You will need a sender agreement (or ICO if on single stack) which has "Sender uses virtual receiver" checked. The interface and namespace on the sender agreement should match the values in the ResponseOneWayBean of the previous step's sync receiver channel.
Yes, you will need 2 HTTP_AAE channels, firstly the receiver HTTP_AAE for the sync request, and secondly the sender HTTP_AAE (used in above sender agreement) for receiving the sync response.
Regarding the usage of the HTTP_AAE on the sync side, I haven't used it before so I'm not sure if it will work or not per the comment on the blog. If after setting up the configuration above and it does not work, I would suggest you switch to SOAP instead. I've used SOAP and it works perfectly fine. On the SOAP channels, check "Do not use SOAP envelope" and it would behave like an HTTP POST.
Rgds
Eng Swee