Thanks Tim,
I revisited that section of the config, and changed the subnet from the user VLAN to the switch VLAN, and that partially fixed the problem. Now the error I get is : "wmi: Skip probe since required port(s) [135] are not open on host. Username is empty in the request"
This persists if windows firewall has been configured to explicitly allow WMI on 135, and even when windows firewall is turned off. The username for the subnet is definitely not empty.