ok.. fault was mine (as usual :-)..
we shared the created client stub between our threaded ejb's. It seems the client is not threadsafe.. when we created dedicated clients for each ejb we got no more errors.. (should it be threadsafe?)
another (minor) problem that showed up was that looking up a ws client does not seem to be thread-safe, ie jndiContext.lookup( "my ws service-ref" ) fails if run in multiple threads. bug?
A jaxrpc Call object is a stateful construct, hence it is not threadsafe. Each client needs to use its own Call.
The lookup of the Service should be threadsafe. What is the exception that you are seeing?