It seems that I have solved this problem by using DOCUMENT style instead of rpc.
JBossWS doesn't like the "ns1:" prefix on the operation or parameter names. I found this with using the MSFT SoapToolkit.
The true reason is, that with RPC style the RPC parameters are not namespace qualified according to the abstract contract in WSDL.
So, the above SOAP message is invalid with respect to WSDL. The exception message should be clear enough.