The reason your Webservice client call suddenly fails is because of a bug in JBoss WS with JBoss 4.x
I haven't tested this on JBoss 5, so I cannot for sure exclude this bug in JBoss 5. However, the bug seem to be fixed in jbossws-2.0.
The solution to this problem is to
Copy the three jars from <JBOSS_HOME>\server\default\lib to <JBOSS_HOME>\lib\endorsed. jboss-jaxrpc.jar jboss-jaxws.jar jboss-saaj.jar
More information can be found here