I saw some bugs reported with an error in the config module for the DHCP Server. I suposse you've changed this settings in newer versions, but the fact is DHCP Server config parameters are still wrong for the version 4.
If it helps, now I give you the right options, they should be changed from these:
To these ones:
Can you please fix it in the next version? Thanks a lot!
OS version: Ubuntu 12.04.3
DHCP Server package version installed: isc-dhcp-server 4.1.ESV-R4-0ubuntu5.9