I’m not sure where would be a good place to post this, but I figured I would get this up here in case it can help anyone else. There apparently was a new update in the xenial netboot image that now segfaults on installation (https://bugs.launchpad.net/ubuntu/+source/glibc/+bug/1816846). They are not anticipating a fix, and if they were to give an official fix, it would be to use the xenial-updates version. So I had to update my Ubuntu 16.04 OS release to xenial-updates to force it to use the latest, updated version of the netboot image. Once I had done that, I had also experienced an issue where the latest kernel version did not have a linux-signed-image deb package. I was able to set the preseed-kernel-image variable to the latest one with a valid linux-signed-image package, which then resulted in a successful installation. Hopefully this helps someone else who is searching for a solution!