If PUMP gets an OFFER or ACK back without a server id option it bags everything. While RFC 2131 is clear that OFFER and ACK must have the server id, some simple DHCP servers (such as those found in Ascend boxes) do not return that option. A simple workaround is to use the broadcast address if the server id isn't present.
There is a patch that fixes this and other problems in pump (as well as reporting errors in syslog) at http://www.cse.ucsc.edu/~breed/pump.patch.
Done in CVS (we'll have a package soon). (thanks for the patch)
This should be fixed in the latest errata release.