I have a WDS server setup for use with MDT2010, and everything was working fine until this morning. Now, whenever I try to boot from LAN, I get an error:

Downloaded WDSNBP...

Architecture: x64
WDSNBP started using DHCP Referral.
Contacting Server: 10.50.10.12 (Gateway: 0.0.0.0)
No response from Windows Deployment Services server.
Launching pxeboot.com

My PXE Response Policy setting on the WDS is set to this:

[ ] Do not respond to any client computer
[ ] Respond only to known client computers
[o] Respond to all (known and unknown) client computers
    [X] For unknown clients, notify administrator and respond after approval

The odd thing is that if I clear the approval option (so any computer gets a response) it works fine.

I have delegated permissions on the AD OU to the computer object WDS is running on, but that doesn't seem to have helped.

As it works without approval, I can only assume my DHCP options are fine and this is some sort of AD permission problem.

(Server is Windows Server 2008 SP2)