Looks to me like the fault is occurring on the client side, failing in trying to deserialize one of the returned parameters.
There are several posts on this forum recommending that SOAP-RPC is to be avoided, primarily because of this type of interoperability issue.
You can review the session logs to see if the client is hitting IS. Look for the username that the client is using (hopefully they’re not using one of the built-in accounts like Administrator or something).
Would strongly recommend changing to use SOAP-MSG. There are threads on the forum that describe how to do so.
#webMethods#soa#API-Management