after I have deleted the only line from the RHQ_STORAGE_NODE table by hand, the node came up, and so does the agent!
rhq=> select * from RHQ_STORAGE_NODE;
id | address | cql_port | operation_mode | ctime | mtime | error_msg | maintenance_pending | resource_id | resource_op_hist_id
------+--------------------+----------+----------------+---------------+---------------+-----------+---------------------+-------------+---------------------
(1 row)
rhq=> delete from RHQ_STORAGE_NODE where id=1001;
DELETE 1
In no time there is the storage node running, and also there are the resources in the discovery queue.