Error is in method dnMapSets (or its uses). It tries to merge 2 hashset which looks like that
Map(QueryNodeDn ->
Map(networkInterfaceLogicalElement ->
HashSet(nodeId=dc23eced-3b84-4f63-9ef8-0c779ddd9fdc,ou=Nodes,ou=Accepted Inventories,ou=Inventories,cn=rudder-configuration, nodeId=9faeba1e-9e11-4cf6-aa66-48c3b70d38dc,ou=Nodes,ou=Accepted Inventories,ou=Inventories,cn=rudder-configuration, nodeId=df4adc78-34e2-4a9b-a37e-700cb6836e97,ou=Nodes,ou=Accepted Inventories,ou=Inventories,cn=rudder-configuration, nodeId=435a0078-9549-48af-b7ba-397be72303e3,ou=Nodes,ou=Accepted Inventories,ou=Inventories,cn=rudder-configuration, nodeId=effb022d-143a-4ee0-9ed8-14eedc542255,ou=Nodes,ou=Accepted Inventories,ou=Inventories,cn=rudder-configuration, nodeId=96fe2b7e-832c-4b61-be82-d41d03ca090f,ou=Nodes,ou=Accepted Inventories,ou=Inventories,cn=rudder-configuration, nodeId=f1d3a9fd-6a8a-471d-b3d2-8eeadcae88e2,ou=Nodes,ou=Accepted Inventories,ou=Inventories,cn=rudder-configuration, nodeId=1a71fb11-6124-462b-8934-eb3bb6e7a53a,ou=Nodes,ou=Nodes,ou=Accepted Inventories,ou=Inventories,cn=rudder-configuration, nodeId=60d1e58b-1864-4a68-911e-eddca15f59df,ou=Nodes,ou=Accepted Inventories,ou=Inventories,cn=rudder-configuration, nodeId=0c664afb-399d-4449-9132-14be3a7d5bae,ou=Nodes,ou=Accepted Inventories,ou=Inventories,cn=rudder-configuration),
group:LDAPFilter(nodeGroupId=all-nodes-with-cfengine-agent) ->
HashSet(nodeId=43ffdce6-8759-4a9e-9d13-5a85279cbe0c,ou=Nodes,cn=rudder-configuration, nodeId=1df7cfab-747f-46a5-836e-085addf4b670,ou=Nodes,cn=rudder-configuration, nodeId=aad622d4-f11c-44e6-9a6b-25ac5e528d8b,ou=Nodes,cn=rudder-configuration, nodeId=0161cb3e-e82f-4d14-881f-34bf10e6ee67,ou=Nodes,cn=rudder-configuration, nodeId=4f8d197c-4122-40b8-99e3-8d4b2cd6f7cb,ou=Nodes,cn=rudder-configuration, nodeId=9a88290d-f859-4b35-babb-c68791895cda,ou=Nodes,cn=rudder-configuration, nodeId=7d3b5e3e-e6ca-4d56-8849-c90d82139f71,ou=Nodes,cn=rudder-configuration, nodeId=de085bc1-f510-46b5-810a-fa73a7307322,)))
Intersection is indeed empty, as in one case we have
nodeId=dc23eced-3b84-4f63-9ef8-0c779ddd9fdc,ou=Nodes,ou=Accepted Inventories,ou=Inventories,cn=rudder-configuration
and the other
nodeId=43ffdce6-8759-4a9e-9d13-5a85279cbe0c,ou=Nodes,cn=rudder-configuration