Hi Erva
This is quite weird, your synchronous request don't seem to show any error up to the Call Adapter step (which is past the Receiver Agreement part), but the error is shown on the synchronous response part.
Running out of ideas here, but can you try the following:-
i) Can you check if the synchronous request reaches the backend system (which I'm assuming it using proxy since you mentioned above it's SOAP to XI scenario.) Can you check on SXI_MONITOR on your backend system if there are any corresponding message for it?
ii) Can you provide screenshot of the OutboundBinding section of the SOAP Header on the Call Adapter step?
iii) Have you tried checking all cache related issues? Here are some How-To handle cache documents. It's a bit old from the XI3.0/PI7.0 days, but a lot of the content are still relevant.
How to Handle Caches in SAP Exchange Infrastructure 3.0 (NW2004)
How to Handle XPI Caches in SAP NetWeaver 2004s (NW7.0)
Rgds
Eng Swee