it has been solved.
In previous installations wihout a cluster the Queue Definitions within the WAS were always configured with a base queuemanager. Empty this field and voila, it worls.
we're implementing an ESB infrastructure with WAS5.1 and MQ5.3. Each WAS node has his own local queue manager instance on the same box. (QM_WASn). JMS accesses QM_WASn in client mode (authentic ...