Hi, came across this
thread will spending a miserable couple hours tracking down something similar.
Our problem turned out to be classpath - ensure that webserviceclient.jar from same weblogic server version is being used. Should suffice, but if weblogicservice.jar and weblogic.jar are required for some reason, ensure they follow the former in that order.
The context of our problem was running client within Eclipse, and there was a weblogic.jar of unknown provenance in the JRE path.
Removed this and problem went away.