Hi Armin - May i know the URL your party is using to send the messages?
It should have the quality of service in it at the end.
Like
https://host:port/sap/xi/adapter_plain?service=<<CommunicationComponent>>&namespace=<<NameSpace>>&interface=<<Interface>>&qos=EO
Also i never tested but even the plain HTTP adapter does have https without client authentication option.
Configuring the Sender Plain HTTP Adapter (SAP Library - SAP Exchange Infrastructure)
