If it is nonblank, it is the user identifier that is to be used by the message channel agent for authorization to access WebSphere MQ resources, including (if PUTAUT is ...
The repos mgr reads SYSTEM.CLUSTER.COMMAND.QUEUE. For it to crash some action by the user must have caused a bad msg to be put to the queue.
Maybe some apps put non-PCF msgs to the queue, or some ot ...