Did this ever get resolved? We are having the same issue with one of our clients, and seeing the same error message. One thing that I noticed in our environment is that the sourceIP of dhcp request is coming from the originating vlan interface, which is actually a vlan that is terminated by the controller and not routed in the internal network. The DHCP is being forwarded through a different vlan on the controller which is connected to the wired network. The problem is, the server is replying with a dhcp offer, but the network doesn't have a route back to that subnet.
This seems like an issue with how the DHCP helper is forwarding the DHCP request. With the limitation that has been set on the number of DHCP IP's that the controller can issue, but the desire to still "hide" client vlan's behind the controller and not have to route the traffic internally, it is becoming necessary to send the DHCP to an external server ... but if I don't want to route these "hidden" subnets, how will the server ever get the address back to teh client if the source IP of the DHCP request is the IP of the client vlan? That is what the "relay agent IP" is with the DHCP request. The actual src IP of the packet should be whatever interface is used to route the traffic to the DHCP server.
Or am I missing something?