I'm running an SAP PI/PO scenario and having a problem with finding the response message (the error one).
Scenario: PROXY (class) -> SAP PI/PO 7.5 -> SOAP (wsdl)
The error when executing the scenario is:
Transmitting the message using connection SOAP_http://sap.com/xi/XI/System failed, due to: com.sap.engine.interfaces.messaging.api.exception.MessagingException: Error encountered while executing mapping: com.sap.aii.af.service.mapping.MappingException: com.sap.aii.utilxi.misc.api.ResourceException: Could not determine mapping steps for message 3b14b3f8-0860-11ed-a034-000001795062
The problem is that I cannot find the message (3b14b3f8-0860-11ed-a034-000001795062) in SAP PI monitor. It's the response message (who is causing the error), as the request message I did find in the monitor and it's correct.
Where I can find it? Why is not showing?
From what I read, this problem happens with several reasons, I just want to find this message.
The webservice works correctly, I test it in SOAP UI.

To find the response message (which contains the error), we have to first activate the Integrated Configuration log (it's not active by default).
To activate the log, we have to follow these steps (more info: https://blogs.sap.com/2012/11/06/message-staging-and-logging-options-in-advanced-adapter-engine-of-pi-73x/)
Now, in the SAP PO monitor (Message monitor), by clicking the "Related Messages" option, we can see the response message error.
In this case, the error was:
In which the Action was empty.