PROBLEM
When updating RBL entries under System->Relay->RBL Filtering on either cluster node, the setting is not replicated on the other node.
SOLUTION
This is a bug in the upgrade script for existing cluster configurations. To fix delete all entries from both nodes without Applying Settings.
Re-add entries in desidered order and state (Active yes/no) on one node and Apply Settings.
The other node should now be ok.