I'm fairly certain that the error was caused by a problem in dhcpd.conf Each time I add a new node, when dhcpd.conf is automatically re-written, the allowed MAC addresses are changed to an incorrect value. I had added a new node just before running this experiment, and neglected to manually fix dhcpd.conf
Hmm, this indicates a more serious problem ... perhaps confusion between what is an experimental interface and what is your control interface. ie: the DB is wrong when you add the nodes? Check the interfaces table to see if the MAC assigned to the control interface is correct. Lbs