Netware 6.5 SP3 server running DHCP.

We have a problem that when our WAN link goes down the local clients in the office are not able to connect to the server.
The local server runs dhcp and the dns server is on a remote server in the head office.
When the link is down the DHCP server gives the following error :-
DHCP UDP server port (67) registration failed.
When the link comes back up the local clients are then able to get an IP address and then connect to the server.
The local server is always up during the wan outages.
Any ideas what could be the problem?