Hi Jean-Philippe,
I have been facing the same issue as Sai and you. My PO system, the usage type is XPI instead of AEX.
Also, I tried the work-around that is working for you: Application Communication -> General & Messaging tab -> Transfer Protocol changed from HTTP header to SOAP header, but even this change is not working for me. Did you change any other parameter too along with this change.
Please let me know in case I am missing any configuration or in case any parameter change is required to make it work.
Thanks
Shivangi