Inbound Web Service calls get error "Reliable Messaging(RM) configured, but no Message ID..."

We are setting up the Communication System and Communication Arrangement "Business Partner Replication from SAP ERP".

For protocol, we are using Type 5 - WebServices. See screenshot below.

Our error is that when we send webservice calls to our C4C, we get messages:

1. Web service response is "Web service processing error; more details in the web service error log on provider side"

2. Backend error messages is "SRT: Plain SOAP: Reliable messaging (RM) configured, but no Messaging ID and no WSRM assertion provided"

We have sent messages from PI and also independently thru soapUI.

So it seems the C4C in the backend is stuck on the RM messaging protocol setting.

Appreciate any and all assistance.