You say your DHCP server is connected to the same switch – but what about your WDM server? Are all devices and server on the same subnet? If not, you’ll need a helper address, even if they are the same physical hardware. Any router VLAN’s configured? Just throwing out anything I can think of that I’ve run into in the past.
The only subnet I can connect to that doesn’t need a helper is the same as my WDM server. All others require it.
And for the record; we don’t have ANY dhcp options configured for WDM (at least in the Active Directory DHCP scopes), XPe doesn’t need them. Our environment is all 9150’s and V90L’s, with WDM Enterprise 4.5.3.