Normally this is due to there is 2nd DHCP server in the same LAN environment.
The 2nd DHCP server provides an IP address to the DRBL client, however, it does not provide NFS service. For DRBL client, you have to make sure there is only one DHCP service, i.e. the one from DRBL server, in the LAN environment. To solve this, you have to disable the 2nd DHCP server, or isolate your DRBL environment from that DHCP server.
Or you can check this (not easy to achieve):
For more info, please refer to: